Mostly because Muslims are painfully dull to play. Nomads at least have constant action going for them.I would put good money on more people playing as nomads than even muslims. A LOT of the let's plays, WCs, and AARs are nomad playthroughs.
Paradox collects statistics like this i'm sure it's out there somewhere.
If they are not included I probably won't play anyone east of Poland until they are for sure. I'd expect it to be an early if not the first DLC.