It is a point.
My own thoughts on the matter. First, if it is famous because it is brought in Rashi, then perhaps it is because it appears first, and people stopped reading. Or else because mekoshesh etzim has eitzim in it, so that it is recognizable. But who in the world were the ma'pilim?! Or else because the way they are exposed to it is not directly in Rashi, who in turn gets it from other sources, but rather in derashot about how if the Bnei Yisrael had only kept Shabbos two weeks in a row -- together with this identification, whereas the other sin is less likely to come up.
Also because this identification helps in two different regards. The daughters of Tzelophchad say that he died in his own sin, without specifying what it was, and so different answers can help fill in the blank. But also, the mekoshesh etzim is not identified, and a closed-canon approach demands that we put in some otherwise known personality. Thus, Tzelophchad fills a role here. But for the many who wished to enter Eretz Yisrael anyways, since it is a group, there is not as much of an impetus to identify each and every one.
Should we identify Tzelophchad's sin? Should we identify the mekoshesh? It is one prominent opinion that we should not, for the Torah did not reveal it. Indeed, Rashi, in the beginning of the perek, writes:
|of the families of Manasseh the son of Joseph: Why is this said? Has it not already said, “the son of Manasseh”? But to inform you that Joseph cherished the Land, as it says, “and you shall bring up my bones… ” (Exod. 13:19), and his daughters cherished the Land, as it says, “Give us a portion” (verse 4) (Sifrei Pinchas 10), [hence they were of Joseph’s family in spirit], and to teach you that they [who are mentioned in the verse] were all righteous, for anyone whose deeds and whose father’s deeds are not clearly described, but Scripture specifies one of them to trace his genealogy for praise, he is a righteous man the son of a righteous man, but if it traces his genealogy for shame, as for example, “Ishmael the son of Nethaniah the son of Elishama came” (II Kings 25:25), it is known that all those mentioned with him were wicked people. — [Sifrei Pinchas 9]||למשפחת מנשה בן יוסף: למה נאמר, והלא כבר נאמר בן מנשה, אלא לומר לך יוסף חבב את הארץ, שנאמר (בראשית נ כה) והעליתם עת עצמותי וגו', ובנותיו חבבו את הארץ, שנאמר (במדבר כז, ד) תנה לנו אחוזה. וללמדך שהיו כולם צדיקים, שכל מי שמעשיו ומעשה אבותיו סתומים ופרט לך הכתוב באחד מהם ליחסו לשבח, הרי זה צדיק בן צדיק. ואם יחסו לגנאי כגון (מלכים ב' כה כה) בא ישמעאל בן נתניה בן אלישמע, בידוע שכל הנזכרים עמו רשעים היו:|
so the genealogy is given for praise. Contrast that with the identifications of Pinchas, Cosbi, and Zimri in the beginning of that parsha, Pinchas. (See my discussion here.) Right at their deeds, they are identified and their genealogy is identified. This shines a spotlight on them and their family, for praise or for shame. Yet the mekoshesh was a sinner and unlike Zimri and Cosbi, was not identified. If the Torah concealed it there, then perhaps it is inappropriate to identify the person. And from the opposite end, the Torah just records the daughters of Tzelophchad saying that he died in his own sit. It seems as if going out of its way to avoid specifying the sin. If so, despite inclinations towards the closed canon approach, perhaps we should leave it unspecified.
Furthermore, perhaps the closed-canon approach should not be used here, and there was some other sin involved. It might not be justified to identify any sin of the ones which happened to be mentioned in the Torah. Perhaps it is an open-canon sin!
Shadal says that his sin was that of everyone else's sin, and thus he was condemned to die in the wilderness. Thus, Shadal writes:
כי בחטאו מת:הוא מת בחטא שחטאו כל שאר היוצאים ממצרים, שכולם מתו במדבר, כמו שגזר ה' ( למעלה י"ד כ"ט ) במדבר הזה יפלו וגו' (שש"ם).
This might strike you as strange. After all, the pasuk (in Bemidbar 27) states:
|ג אָבִינוּ, מֵת בַּמִּדְבָּר, וְהוּא לֹא-הָיָה בְּתוֹךְ הָעֵדָה הַנּוֹעָדִים עַל-ה', בַּעֲדַת-קֹרַח: כִּי-בְחֶטְאוֹ מֵת, וּבָנִים לֹא-הָיוּ לוֹ.||3 'Our father died in the wilderness, and he was not among the company of them that gathered themselves together against the LORD in the company of Korah, but he died in his own sin; and he had no sons.|
If they are taking him out of the congregation in the first half of the verse, why would they put him in a different congregation? That appears to be the intent of "his own sin." Shadal has his answer, in which it is important specifically to take him out of the congregation of Korach, who forfeited all their property.
Rashi would appear to be at odds, for he explicitly takes him out of the mitlonenim, complainers, writing:
|but he was not…: Since they were going to say that “he died for his own sin,” they had to say that it was not for the sin of those who grumbled, and [that he was] not in Korah’s company who incited [the people] against the Holy One, blessed is He, but he died for his own sin alone, ad he did not cause others to sin with him (B.B. 18b, Sifrei Pinchas 13). R. Akiva says, He was the wood gatherer [see 15:32], and R. Shimon says: He was among those who ascended [the mountain] defiantly [see 14:44]. — [Shab . 96b]||והוא לא היה וגו': לפי שהיו באות לומר בחטאו מת, נזקקו לומר לא בחטא מתלוננים ולא בעדת קרח שהצו על הקב"ה, אלא בחטאו לבדו מת, ולא החטיא את אחרים עמו. ר' עקיבא אומר מקושש עצים היה. ור' שמעון אומר מן המעפילים היה:|
So not only was he not in Korach's company, but he was not in the company of the complainers. He derives this from "his own sin" that the daughters of Tzelophchad mention. I suppose since all those died from this collective sin on all of Israel.
This also, by the way, helps along the first idea, that he was the wood collector. For that is an individual sin. Meanwhile, those who ascended defiantly would seem to be those who earlier grumbled (though if so, their death came sooner); and even if not, it is a collective rather than individual sin.
Perhaps the way they arrived at this sin was by looking sequentially backwards. Skip Zimri, because that would be extremely cheeky here. It is not Korach, so go further back. Then we encounter the mekoshesh etzim, in perek 15. And immediately beforehand, towards the end of perek 14, we encounter those who defiantly ascended.
The midrashist in my prefers to mekoshesh etzim. And this for three reasons. (1) The closed-canon approach would demand a person for the mekoshesh. (2) As mentioned above, they said his own sin, and those who ascended defiantly were a collection of people. (3) This one I have not yet mentioned. The daughters of Tzelophchad are famous for posing a question which Moshe did not know how to answer immediately. Thus,
It is therefore fitting that the same occurred for Tzelophchad himself. He performed an action and Moshe did not know the proper response immediately. Thus:
It is then fitting, and is history repeating itself. I like such neat parallels in my midrash.