I think at one time or another we each reach a place where we stay with the hobby or leave it.
So why it that, I think the big one is lack of time, followed by bad weather or not knowing the sky as well as we wish we did or even not having the dream scope we think would keep are interest.
I don't think there's a single person who has ever left the hobby because they couldn't buy their dream scope. There might be some who have said that, but they wouldn't have lasted in the hobby no matter what telescope(s) they owned. Most likely it's a sign of unrealistic expectations -- no scope, no matter how dreamy, would have made astronomy glamorous and effort-free as they were dreaming.
There are a fair number of people who toy with the idea of astronomy but never really get into it -- never get over that initial hurdle. No doubt this is true for the great majority of people who buy telescopes -- and even more true for the ones who are receive them as gifts. I don't think you can really say that these people are leaving astronomy, because they were never there in the first place.
Mind you, such people might yet come back again. I did. I was given a scope as a child, but never really figured out how to use it, so that was a false start. Then I explored the scope through 7x35 binoculars when I was 20, but my interest became dormant for several decades. And finally, I purchased yet another scope and really got over that first hurdle.
Since then, my enthusiasm has waxed and waned, but unless I go blind I will never stop being a stargazer. Even when I have no access to a telescope, I still go out at night to check up on the positions of the planets and make sure the stars are still in their familiar constellations. (So far, they always have been.)
I think that's quite typical. I do know a fair number of people who became adept at using telescopes, but then drifted off into not using them. But they could easily restart at any time, unexpectedly.