He Wanted to Oppose the Early Morning Waking:

The Chasan Sofer zt"l writes that Bilaam got up early because he wanted to arouse an accusation against Klal Yisroel. He wanted to show that they are lazy and don't get up early to serve Hashem. Hashem responded by saying that Avrohom also got up early; therefore, his claim is invalid. He adds that this may be the reason why the Avos enacted the three daily tefillos, so that we will always rise early to serve Hashem. The Chidushei Harim zy"a (quoted in Sefer Likutei Yehuda) further explains that the complaint against Klal Yisroel was not really so great because Bilaam had the yeitzer hara on his side, which was helping him to do his wicked actions. When one does mitzvos, however, the yeitzer hara tries to find ways to trap him and obstruct what he wants to do. Thus, the complaint was not really that strong.