I wonder why Phil never played an acoustic base....it seemed like he'd have been a natural on that instrument.

Phil Lesh "did dabble with an acoustic bass guitar but he wasn't a trained player on it. He's mentioned to have used an acoustic guitar before concerts for practice, but he preferred not to use a 6-string acoustic because it made his playing feel too loose, making it harder to transition to the bass."

He was trained as a trumpet player and had never before played bass, so he learned "on the job" and "was free to develop his own style." (Wikipedia)

(Unlike other bass players, he played a six-string electric bass guitar)