Bosnia was the home of a "heretic" brand of Christianity which existed alongside, and often fought with the mainstream Catholic and Orthodox versions. The last few Kings in Bosnia especially contributed to the hazy situation by trying to enforce Catholicism on the heretics, so when the Ottomans conquered a country they found a population which was a lot less closely tied to their Christianity than, for example, the people in Greece.
IIRC Albania was Islamized much later than Bosnia; the causes are less clear, but Dervish activity was an important part of gradually spreading and fortifying Islam there.