I don't believe there's been a stated reason, but we can surmise several
Newer aircraft have newer, more efficient engines meaning typically lower noise and less pollution.
Newer aircraft are typically better maintained. Obviously this isn't always true, and some older aircraft can be very well maintained, but it's easier to just put a blanket cut-off policy in place, rather than waste time on a case-by-case basis. It also gives the perception to the public that a safety policy has been put in place.
Even if they aren't better maintained, newer aircraft are less likely to have developed faults. Again, there is no guarantee, but it provides a lower risk profile.
And as stated in another answer, there is possibly a degree of protectionism - local airlines in that part of the world tend to be fairly young with modern fleets, whereas some of the larger carriers elsewhere have older fleets. I'm not convinced this would be an effective tactic, however, as the likes of Lufthansa, British Airways etc have enough new planes to service routes to the UAE etc
And finally, older aircraft tend to be snapped up by less.... Conscientious... Airlines. Age limits are a good way to force these types of low-cost, potentially 'iffy' airlines out of contention if you don't want them flying your people in and out of your airlines.