Some of it is probably tradition and aesthetics. Decades of experience have trained a lot of cyclists into what a hub "should" look like. And that design may be easier to create or assemble into a wheel.

When you have a rim with 28, 32 or 36 spokes, there is probably no benefit to using straight-pull spokes.

I've ridden lots of wheels, though, with low spoke counts with j-bends, and they've worked well.

It's often easier to replace a j-bend spoke when one breaks. You don't have to partially disassemble the hub, which is necessary with some straight-pull designs.