I don't think there's a reason, which I think is the saddest reason. Growing up and drifting apart. Tried many times to fire up conversations but don't know where to start.
If that's the case, my gut feeling is you may have been friends out of convenience. For friends I haven't talked to in decades; we pick up like no time has passed at all.
Nah these things are funny like that you can end up just like this with very close friends, it's not inevitable but you also can't reasonably predict which friendships are like the ones you describe that can survive a long hibernation and ones that for all the will in the world enter a freeze from which they never fully thaw.
Maybe, I'm biased on all my friends from highschool onward. We lost touch because of moves and family obligations...but when we find each other again is is like no time has past.