The main reason is to do with heat. Even when it is used under normal load, the amd apu will produce a lot more heat the the intel counter part. But its performance is generally greater than a lot of igpu's.
The problem is that most people that want power will prefer a cooler laptop as it results in the laptop being quieter. Also people really serious about power will get the top of the line dgpus from nividia which outperform the amds by a lot.