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.
This. I've never been good at making friends but find the few I have are people I can genuinely rely on. I'm really tired of the superficial conversations and anything that involves one-upmanship. I think as you get older, have kids etc you have so little genuine free time for yourself that you become far more choosey with who you share that time with.