I think that one of the main reason for those oscillator speed is they're not as popular as the other AND, of course, it's bot as this easy to implement timing with...
Some maths
4MHZ 1 uSec cycle
8MHZ 0.5 uSec cycle
10MHZ 0.4 uSec
12MHZ 0.33333333 uSec
20MHZ 0.2 uSec

SO when you comes up with a 11.059MHZ OSC you got
11.059MHZ
3,6169635590921421466678723211864e-7 cycles... How on earth can you get accurate PAUSE 1000 without any trick??? Yeah PBP could implement them but as they're not the main one we use... so i guess it just don't worth NOT ONLY FOR MELABS... in almost compiler i use.