Not long before Fender started making basses, they tried making Telecasters/broadcasters without truss rods in the neck, and...it didn't go well - necks started twisting from the tension of the strings. A few years later, they start making bass guitars, and...the longer the neck, the less stable it is, and they migh very well have been worried about that. My guess is they put the least amount of frets on it that they thought they could get away with, in order to avoid more neck problems, especially given the increased tension of bass strings. Just speculation, but I'd be surprised if they didn't think of that as at least one consideration. From the introduction of the Precision to the Jazz, in that time period, people were not doing solo bass guitar, so I'd guess they didn't get a lot of negative feedback about more frets, and figured that enough frets for a P bass was enough for a Jazz.