Each addon is usually only developed by one person, or a small group of people. So all it takes is that one person losing interest to stop it being updated. That could be because they've stopped playing ESO, because they stopped making/updating addons or because they decided someone else's addon did it better and theirs wasn't needed any more.

I don't know, but I suspect in some cases they just decide updating it is pointless. A few of my addons like Binder and Banker Shutup haven't been updated in months, but they still work. It's probably not worth the hassle of changing the API version and uploading it again to get rid of the 'out of date' tag when it still works (and if it bothers users it's a really simple change to make yourself). Especially if the person doing it just made the one addon because it's something they really wanted rather than being a regular addon creator.