This linked answer took me a long way towards solving this riddle but didn't quite answer it. According to it, Shrimad Bhagvat clearly states that the purpose of Vishnu's incarnation as Buddha was to delude the people who are envious to the Gods (read athiests):
tataḥ kalau sampravṛtte sammohāya sura-dviṣām
buddho nāmnāñjana-sutaḥ kīkaṭeṣu bhaviṣyati [SB - 1.3.24]
This view is also prominent among many other schools of Hindu Philosophy.
Now I'm not challenging the existing scriptures, but simply trying to understand why. What sense would it make to "delude" some people by denying moksha, if they aren't going to get it in the first place? More over, seeing what Buddha has brought into the world (non-violence, meditation, etc.), that doesn't seem to be delusional at all. How do you explain that?