The unfortunate fact of the matter is that religious vocations in the Catholic Church are way more rare than they used to be. The church sometimes struggles to have enough priests to meet the needs of the parishes, let alone having enough monks to keep up all the monasteries.
Since there aren't enough monks, the church would have to pay people to come in and maintain the place - but they can't really afford that. So, either they let these beautiful places crumble and decay, or they sell them to someone who will hopefully be able to take care of it. Between the two, the latter seems like the least bad option to me. I certainly wish that these places could stay as part of the church and not be some billionaire's home, but I would rather they be a billionaire's home than go to ruin.
