why did the 515 prayers of moshe not help him?

Shalom, Thank you for your question. You are, of course, referring to a Midrashic comment on Devarim 3:23. The verse states that Moshe entreated Hashem to be allowed to enter the land of Israel with the Jewish people. The word used in Hebrew is “VeEtchanan”, which has the gemaritrah, numerical value, of 515. From here it is learned that he prayed 515 times. You ask why this did not help him? Firstly, we cannot be certain that it “did not help him”. It did not let him enter the land of Israel – but may very have helped him in other areas. We believe that all pray helps. It just not necessarily get answered by Hashem with a “yes”. As to why Moshe was not allowed to enter the Land of Israel with the Jewish people, there are various explanations. Rashi states that it was because of the sins of the Jewish people. Others explain that it was a punishment for not following Hashems command when drawing water from the rock (this is the simple reading of the text). Still others explain that had Moshe led the Jewish people into the Land, we would have been on a spiritual level that we were not ready for. I hope this is a good start to your further studies. Blessings.