Albanians are another ethnic group that is predominantly Muslim.

As for Bosnia, we need to understand its history long before Ottoman conquest. After Slavic tribes migrated to Balkans, Christian missions from Rome and Constantinople were sent to their lands in an effort to christianise them. This lead to establishment of boundaries between See of Rome and See of Constantinople. After the Great Schism of 1054, the lands in Croatia and most of Dalmatia fell to Roman Catholic church, while Serbia fell under Orthodox Church. Bosnia, while nominally Orthodox, was kind of a "no-mans" land between both faiths due to its remote location which made establishment of any church network very difficult. This made Bosnia a welcome refuge for "heretic" faiths such as Cathars and Bogomils. Hungarians used this as pretext to launch crusade against Bosnia. This made Rome very unpopular in Bosnia, and that led to establisment of Bosnian Church. So now you had Catholic Church, Orthodox Church, Bosnian Church and Bogomils all coexisting there. The Bosnian Church and Bogomils were declared as heretics by both Catholics and the Orthodox, so now when the Ottomans invaded, they had an easy job converting those considered as heretics to Islam since they had no support from Rome or Orthodox community.