The Medrash (Devarim Rabbah 7:10) relates: "Rav Shmuel bar Yitzchok said: Moshe was close to death and the people did not daven for him to be able to enter the land. He gathered them and began to rebuke them. He said, 'One (man) redeemed 600,000, and 600,000 can't redeem one man? Did 'Hashem not give you a heart to understand (Devarim 29:3)?' He told them, 'Do you not remember all that I did for you in the desert…'"
The Sefas Emes (Likutim) states that the reason Klal Yisroel did not daven for Moshe was probably because they thought that if his prayers didn't work, theirs surely wouldn't. However, they were incorrect because the truth was that he was only missing "one more prayer", which is why Hashem commanded him to stop davening (Devarim 3:20). If Klal Yisroel had prayed that tefillah for him, it would have worked. Moshe, however, was unable to ask them directly to do it, as this was also forbidden by Hashem as part of his command to stop. Instead, he hinted to them that if they had prayed one tefillah for him, he would be able to enter the land.
