Because if the aircrafts final requested level is FL380 but they can't take that till past the Saudi/Bahrain boundary due to being too heavy that level is useless to us, cos we need to have everyone stacked up and procedurally separated for their trip to Europe via Saudi "Non-Radar" airspace.
From a limited Area perspective my understanding is that the aircraft on startup pass on their requested level to Dubai Tower. That level is then passed on to us, which is then dependant on other traffic passed on to Bahrain, which then passes it on to Saudi Arabia. Therefore with the volume of departures we get from Dubai and Abu Dhabi for Europe, we don't have time to decide what is a likely final requested level for an aircraft type and what isn't. What is passed to us, is what the aircraft gets. If traffic and workload permit we will endeavour to get them higher but that is not always possible.
Due to congestion on that route it is not unheard of for B747's to be FL260, FL240 or even FL220 final level from Dubai to Europe due to all levels occupied above.
P.S The highest requested level in our airspace is pretty useless as we are stacking the traffic up for Bahrain and Saudi, so the level they request in our 100 NM's or so is pretty irrellevant.