Keep in mind, it was an era of constant fragmentation of Balkan states into smaller hegemonies and all these independent kings, despots and hegemons were seeking external help to handle their civil disputes.
It comes to mind the case of John Kantakouzinos, one of the contenders of Byzantine throne who find it so easy to ally with Orhan and permit Ottomans this way to have a say in the internal affairs of Byzantine empire. It wasnt difficult for Ottomans to exploit this fact for their own favor.
Ottomans next step was to conquer Thrace, a keypoint who would later help them for their expansion to Europe. Of course this wasnt the only reason. Orthodox populations opposed with fanatism in any further penetration of Catholicism and were willing to do anything and ally with anyone to achieve it. Furthermore Ottomans didnt have to face any serious challenge in Balkans since most Balkan states were in decline, especially the once mighty Byzantine empire.