I'm no expert on making friends but I think it's partly because you are more discerning as you age. You work to the old adage quality over quantity for lots of things, including friends. Therefore superficial friendships are no longer as important iykwim.

If you're anything like me, you don't feel the need to suffer fools and are less tolerant of people's crap. Therefore you automatically, and perhaps subconsciously, filter out a lot of potential friends.