Graphics accelerator cores on SoCs are typically clocked at higher rates only when they're put into dedicated gaming devices. The other times, the chip is being balanced more for CPU operation.
If there's no primary focus on graphics and consequently no high graphics clockspeed, the chips will still likely be run on a small process size in order to make them more efficient in power consumption and silicon area. Keeping the speed low on some of the chip's parts could also ease some pressure on manufacturing yields.
