Even if they adopted German culture, spoke only German, and were completely crimeless, they can NEVER "Assimilate", because they are not Germans.
But there is the the problem: No one wants them to "assimilate". Assimilation would mean they had to even change their look. But what they should and CAN do, is to INTEGRATE. And Most Africans and Asians integrate very well. They speak German very well - in most cases without any accent. They are part of the society, being active in local culture, participating in cultural events and appearing in museums and galleries. Muslims don't.
They are only interested in their slum quarters and mosques, leading their poor lives within their own limited society and rarely you can find some of them participating actively in our society as for example Indians, Chinese or Vietnamese or Africans do. What is so different? African and Asian societies are totally different from ours to, even though they manage to become part of it and contribute positively to it. As soon as an African has Muslim roots, there is no participation and he/she is as filthy and uncultural as a Turk or Paki!
The only exception to that are Iranians - but here the difference is, that most Iranians are actually non-religious and have a good reason to be so!