Why wasn't the Buddha a solipsist?
Solipsism is an extreme opposite of Buddhism. It posits that only the self is truly real. That all that exists is the self. It places the self on a throne.

The Buddha was not a solipsist because he saw and knew the truth: that the self is not real, but rather like an illusion with not one iota of real or inherent existence. Seeing the truth, how could he possibly believe solipsism?