Well, yes, thats part of the reason. Another part of it is, for all intents and purposes a video card consists of a processor (separate from your main CPU) with its own RAM. Not only does the system RAM typically run at a lower clock rate than a video card's RAM, you have to think, that the APU is doing both the work of the the system CPU and the graphics processing. So, the APU has to share it's die space with 2 processors, one for the system, and one for the graphics. Theres an inherent limitation to its capabilities due to space, heat generation, etc.
You can see how its designed here:http://www.custompcreview.com/news/amd-officially-releases-trinity-second-generation-apus/9318/