Tube amps lend themselves more naturally to distortion than solid-state. When overdriven, tubes produce more of the nice-sounding even harmonics, while transistors produce more of the odd harmonics, which don't sound as good. Early solid-state amps distorted very un-musically. Bass is generally played clean and requires a good deal more power, which would require a very large, heavy, expensive, and delicate tube amp, so solid-state became a good alternative for bassists.