I'm sure they used the soft stones because they were easier to carve. But I'm surprised someone in the modern age hasn't come up with a way to treat porous stone like this to make it more weather-resistant, so they don't have to do so much maintenance and repair on these old stone structures. It would be similar to how we use treatments in dentistry to fill the pores in teeth and make them less sensitive and wear longer.
I think that's because of "Denkmalschutz", which means not changing the 'character' of the building, down to not really changing the building materials, even if they wouldn't look different from the outside.
