I think the reason is it's so rare to need more than 20, and when you do it's difficult to access them without a bigger set of cutaways.
Thinking about it though, I guess it depends what you're used to. My U-Bass has only 16 frets, and I find myself really missing the high C note that would be there if it had 17. There are a few things I have to play down an octave to accommodate this, the ad lib bit at the start of Don't Stop Me Now for example