I'll preface this by admitting that I'm no luthier, and what follows might be completely wrong, but....I thought the reason for 22 or fewer frets was that the neck pickup "wants" to be placed precisely where the 24th fret would sit, which is the resonant point for the 3rd harmonic when the open string is vibrating.
If you have 24 frets, the neck pickup is slightly behind the optimal point (closer to the bridge). If you build with 20-22 frets you can put the neck pickup in the ideal location.
If anyone out there actually understands this and can explain better than I can, please jump in.