Upvote:15
Because they are Catholic. No other reason. Doesn't matter if you are a boy or girl. It's very common practise to give children many baptismal names, including Maria. To both girls and boys. Perhaps not today, but when I was baptized 60 years ago, it definitely was.
My parents 'blessed' me with that name too. Being a boy, and attending a non-Catholic school, I wasn't particularly happy with it. (We are in no way related to the Habsburgs!)
It is - or was - a Catholic custom. Not related to Germany or Eastern Europe. I grew up in The Netherlands where this was practised too.
With regard to the Habsburgs, it was very likely also family tradition. Especially as it was a first name for girls.