It's called changing priorities of Real Life. What is hard to understand?
Speaking for myself; my life, the woman I love, my job and my close friends take priority over a game. It does not mean I dont keep informed, or forgot what I learned, but it does mean that what little free time I happen to have goes to other things that make me happy.
Airsoft is a wonderful game, it is a great attraction for gun collectors, but for me that's it for now. There's a few other reasons, but those are personal and honestly nobody's business. I guess it's the same deal for quite a few others.