I think there's more than one factor, but the biggest one comes down to setting expectations, especially in the early days.
Elm does a lot of things differently than other languages, and while I think that overall those differences have been instrumental to its success, especially early on it sometimes wasn't clearly enough communicated when "this is temporarily different because it's a WIP; you can expect it to be more like what you're used to in the future" versus "this is different by design, so expect it to stay different or even to become more different."
Examples of this include typeclasses (a lot of people coming to Elm from a Haskell background assumed Elm would add them as a matter of course), JS interop (by design, it's not a traditional FFI, although there was an unintended backdoor that kind of worked like one; when that backdoor was finally removed, after much communication that in retrospect should have happened at the outset, some of those who had been relying on it were understandably upset), how the project is run (more like Clojure than like JavaScript), and what the release schedule looks like (batching large projects rather than more frequent smaller releases).
I think things are a lot clearer today, but for some it's unfortunately too late to make a difference.