Complex sentence: Both of these monasteries had roofs covered in lead panels, that shined in the Sun, this was the reason people started calling them white churches, and that is why this area is called today Bela Crkva (White Church).
Split into simpler sentences: Both of these monasteries had roofs covered in lead panels. One of the reasons they were called White Churches was because of the way in which the sun reflected off of the roofs, and is possibly why this area is called today Bela Crkva (White Church).