Errrr. Maybe cos the pay (for a jet) is cr@p and people are fed up being away for about 15 nights a month??
The salary might not be great and there might be many night stops, yet not many Middle Eastern operators will give a narrowbody Airbus DEC to a skipper off an RJ/E-Jet, when there will be people out there that do have Boeing/Airbus time on type.
If the above is the case then you will probably earn the same as an F/O in the sandpit but go to the bottom of the seniority list and wait for another command. Not really worth it when viewed in that context! Also being away 15 nights from home is better than only getting home when you are on leave as is the case in the ex-pat world.