You still see them used sometimes in Poland.
Mantilla in Catholic churches in Poland? Really? Where? I've never seen it myself. Only on TV when some head of state is visiting the pope and their wives wear them then. Until Atch wrote about it I didn't even know what it's called ;)
The thing about women covering their heads in church originated (as a specific rule) with St.Paul however that rule more likely reflects Christianity's roots as a Middle Eastern religion.
Yes, this is the origin of this custom and it's still practiced in the Orthodox Church.
What I don't get is why hardly any Polish women wear skirts and dresses compared with other nationalities,
Really? I didn't know that. When I was visiting other countries in Europe I haven't noticed much difference. On the other hand when I'm visiting other countries I'm usually focused more on sightseeing rather than on checking what women are wearing so maybe that's the reason ;)
especially given the Catholic heritage which used to include banning women from wearing trousers in church.
To be honest I didn't even know there was such a ban. I'm not sure whether it was even enforced in Poland, I've never heard of such a thing. I also was never told what I'm supposed to wear when going to church, I always just knew that men should have their heads uncovered, while women can have them covered or not, as they wish (although during summer wearing a hat in a church would be usually seen as a bit strange and rather impolite, I think).
And in all honesty, wearing skirts to church can be a bit painful - you have to kneel from time to time either on cold floor or "footstool" on the bench, so it's better for your knees when you're in trousers.
Also, don't forget that Poland was a communist country for a half a century and all that "Kinder, Küche, Kirche" (Children, Kitchen, Church) and compulsory dressing skirts probably wouldn't go down well as some kind of official dogma for women ;)
As kpc21 wrote, the way you dress to church is rather a matter of common sense and savoir vivre, I don't think I've ever seen in Poland information placards like in Italy where it was shown what I kind of clothes aren't allowed - when I was in Italy some unfortunate tourists couldn't go to Basilica of Saint Anthony in Padua because they weren't dressed in an appropriate way. I could go in because I had long trousers and short sleeves (I made up for this with some cleavage in order not to cook myself and some woman had to comment about that and I felt so embarrassed, I felt like some "wh0re of Babylon" for a few hours xD). For tourists it's a pain because you usually have to walk dressed like this for the rest of the day in the scorching Italian heat.
As for wearing skirts in general - it's as terri wrote - trousers are simply more practical for everyday use. Skirts and dresses can also wrinkle more easily and after a day of wearing it it doesn't look too good. In trousers you can sit in whatever way you like without worrying about showing your panties or the skirt going too high up when you're sitting or sitting on something dirty/cold, etc. I guess skirts and dresses can be seen as sth for more special occasions, when you want to look more elegant or more feminine.
Btw, it's not really why women wear trousers but it came to my mind that an additional bonus is that it's more difficult to rape a woman wearing trousers - there's more undressing, so, at least in theory, a woman has more time to try to escape or hit the rapist on the head with something.
So, yeah, jeans rule ;))