Personally I think there are dozens of reasons why it didn't last. ED is a niche base; just speaking for myself, a large majority of the people I've talked to haven't even heard of ED, much less the show. The fans who are out there are very passionate but there just aren't enough of them.
Now granted, the show may have pulled in new fans. I say that because I'm one of them. I'm a horror fan and knew about ED but never really watched the movies. I saw the promo for the show in season 1 and thought "oh that looks cool". Three season later I'm addicted. So I think there are fairly new(er) ED fans out there that became ED fans because of the show. As a side note, the show will probably always be my favorite and have a special place in my heart
I'm a little torn with Starz. On one hand, the marketing with the show could have been handled differently/better. And right now they aren't investing much in it at all, whereas in the beginning I feel like they did. I feel like it's a pattern with Starz (someone mentioned this on Reddit too); most of their shows don't last beyond 3-4 seasons. Is that Starz fault? Maybe. Probably.
On the other hand, they took a chance on AvED. They were the ones to invest in it to begin with; it's because of that that we were able to get a continuation of Ash's adventures at all. And maybe I'm wrong but I think with Starz they were able to have the freedom to mold the show to their liking and not conform. That's what I worry a little bit with seeing it go to another platform, even Netflix; I want them (the writers/etc) to be able to be true to the ED mythology.
All of this combined with the fact that the show itself is just not easily accessible, I think those are the main reasons it didn't last. Just my 2 cents. I would sign petitions out the wazoo and support the bejesus out of it but I really don't think it's gonna get picked up elsewhere, I think this is it. As much as I hate saying that.
If I were Bruce, I'd have the same attitude. You can't control what you can't change. Yes he was up for playing Ash as long as he could but that likelihood is dwindling. It would probably take more work now to keep it going than not. He had an amazing run, the show had an amazing run and will live on with the movies. So if he wants to retire, good for him. He earned it. \m/