Why does she still care?

My girlfriend of 4 months broke up with her ex of 6 years last year (she was unhappy and had fallen out of love with him) and I was there for her during the time she was still living with her ex and having a hard time with it all (we had an intimate relationship when they broke up and she was still living with him but he doesn't know that). She says she is the happiest now that she is with me.

I don't understand why she still cares about him as a friend? When she found out about him having a new partner a few months ago she seemed angry saying to me "I want to know who she is! Nobody was meant to have him but me" something along those lines, I asked her why she was making a fuss about it because it upset me and she said because while they were in the process of breaking up he tried hard to make it work so she didn't see why he moved on so fast. She seemed weird about him buying a new property too the other week when she found out.

Can someone please help me understand the feelings the dumper has about their ex if they are not in love with them anymore but still care as friends, i understand being with someone for 6 years is a long time but I don't understand because I've never been in that situation before.