Rashi and Targum Yonasan: They wanted to stress that he died for his own personal sin and not for that of Korach, who stirred-up the people against Hashem, or for that of the Mislon'nim, who caused others to sin (or the sin of the Meraglim, who rebelled against Hashem). ¹

Ramban #1 and Moshav Zekenim #1: They were pointing out that he was not in Korach's congregation or the complainers in his congregation, ² who lost their portion in Eretz Yisrael.

Ramban #2 and Moshav Zekenim #2: They assumed that Moshe hated Korach and his congregation more than all the other sinners who died in the desert because they set out to attack him and to negate everything that he had accomplished, and that he would therefore pray that 'he should have no-one who extends him kindness and that no-one should show his orphans mercy, and that the sins of his father be remembered before Hashem'. ³ And they added that he died of his own sin to explain why he died in the desert and did not merit to enter Eretz Yisrael, or they were explaining why he left behind no sons. ⁴

Seforno: They were pointing out that, since he did not die together with Korach's congregation, whose property, which Moshe placed in Cherem, ⁵ was lost from them, they were permitted to inherit his property.

Bava Basra, 118b: "be'Soch ha'Eidah" refers to the Meraglim; "and "ha'No'adim al Hashem", to the Mis'on'nim - who grumbled after they left Har Sinai.

Rashi #1 (citing R. Akiva Shabbos, 96b): Tz'lofchad was the one whom they caught collecting wood on Shabbos. ¹

Rashi #2 (citing R. Shimon ² in Shabbos, 96b): He was one of the Ma'apilim. ³