Im sure every circumstance is different.. but I am in a similar situation. Ill take a guess at why; some dont talk to me as much because I spent too much time wrapped up in my codependency and unstable marriage that I wasnt there for them when they needed me. For some, we just grew apart. Maybe some others got sick of hearing me complain about life. And point simply, Im just not as much fun as I use to be.. growing up sucks! LoL!

If you want some solid answers, ask an old friend or two and see what they say!! But dont be too hard on yourself!!

You might be on to something, I had friends before I was married.

There is no way that all your friends disappear at the same time. There is something with you, and the only person who can figure it out is you.
I, too, had much more friends before I started dating my current girlfriend. And for a long time I didn't realise how dependent I became on her. Gave up my happiness for her. My friends didn't just disappear. It was me who called them less often and spent less time with them. Because my needy girlfriend always demanded my company and she made me feel guilty if I dared to leave her alone. I didn't know this that time, and I'm still working on my way out. But it's a bit better now.
It may or may not be the same situation with you. Just think about it.
Good luck.

Maybe something about your behavior is alienating them?

I wanna say that although I take responsibility in the loss of friendships but honestly when you have someone who calls ur friends to check that u were with them and what u talked about and they get tired of that (and ur not even aware it happened for years after) its hard to maintain any friends. Between thag and allowing him to take up all of my time and energy over the last 15 yrs I'm pretty short on friends now. I try to connect with people. Or reconnect with ones from high school but I'm struggling in that department too.
Good luck. Make sure you are keeping ur selfcare up.

Things change. People change. Needs change. It is good to recognize your own responsibility when relationships change or don't succeed. But don't be too hard on yourself.

Maybe out's not you, but the kind of people you were friends with. You may need better, different people in your life.