The dome of a church like St Stephens is not just "an interior ceiling" - it's the centerpiece of the entire structure, which itself was built as the main catholic church in Hungary.

The dome has a great symoblic meaning in the church - it represents heaven itself - God is literally painted in the center of the St Stephens Basilica dome.

The word "dome" itself came into English from Latin "Domus Dei" or House of God.

Most of the great domes that inspired catholic architecture were concrete - with St Peters in Rome being probably the most important, which itself was modeled after the famous concrete dome of the Pantheon, which still stands after nearly 2000 years.

So given the importance of the dome itself, a church architect wouldn't design one to be just "sufficient for the job" - rather it should be impressive and signal longevity and substance. Concrete was both a traditional material to use, and fulfilled the structural criteria.