Because you don't need more than 20 to make an outstanding bass line!! it's like asking why planes have 2 wings instead of 4, 6 or 8... Or why cars don't have 8 wheels!! you simply don't need that many to make it functional!! A bass can be playable even if it only has 7 frets... Even better than that... FRETLESS!! 0 frets whatsoever and a limitless note range!! You can even pitch shift harmonics by sliding!!
