What caused the majority of Bosnia and Albania's populations to convert to Islam while many other nations that were under Ottoman rule for centuries such as Bulgaria remained devout Christians?
In Bosnia the local religion was influential. It’s disappeared in the meantime because most of its followers converted to Islam, while others converted to either orthodox Christianity or Catholicism. IIRC the local variant of Christianity was somewhat similar to Islam, making its followers more susceptible to conversion.
Albania is a bit more interesting, imo. It was staunchly Christian, rebelling constantly over the years. Eventually the Ottomans got sick of it and doubled down on conversion efforts in areas populated by Albanians