Why do some people keep friends separate from others?

I have always introduced friends to each other. I always would arrange get togethers, I figured if I got on with someone that other friends would get along too.

I have a couple of friends who keep freinds separate from each other, they never invite us out together.
I have one friend in particular that most of her friends she met becuase of me, I met her in college and always invited her out to meet my friends I had made years before college and she got along with them too and it was a lot of fun.

Now she has made a couple of friends through 2 of her ex boyfriends and see's them on their a lot but never invites anyone else! I have met one of them twice and they seem nice and we would get on. But no one else has met her and none of us get invited out with them.

Why do people do this?
Why do some people keep friends separate from others?
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