Off-topic: because the Buddha explicitly favours reason and testability over blind belief. Granted most Buddhists have developed rituals akin to other religions over the millennia (and also a lot of mythology that demands blind belief), but core Buddhism (as given in the suttas) is almost not a religion. In fact calling it a philosophy of life is a better description.

I would define Buddhism as: A form of introspective psychology.

Buddhism makes testable predictions about mental states and proposes systematic mental exercises to test those predictions.