My guess would be that during the 50's Leo was focused on inventing/developing the Telecaster, Stratocaster, and Precision Bass (often under intense pressure from Fender Sales) and thus made it a low priority/delayed fixing the distortion issue on one amp until the early 60's. Especially if he saw this model as not being used often for large stages when other options were available.
Finally, lucky for us, he also used this time to develop a stage amp to go along with his new Bass guitar.
Just my guesses...
