Because it is heavily inspired by Rails, bringing the same problems but taking it to a whole new level which kinda kills most of benefits of Rails.
You still have a tons of code under the hood and lots of opinions (like Rails) but with a bunch of over-engineered stuff on top (unlike Rails). When working with Rails, the amount of code/patterns I need to learn is a trade-off I'm willing to take because there's a bunch of patterns that you pretty much don't need much more flexibility than it offers already (controllers/routes/views-helpers for example), ActiveRecord on the other end, sometimes is a pain in the ass. Rails is already border-line too much shit to just receive a request and concatenate a response but it's "ok" since it's fullstack (connecting to a database, serializing objects, adding headers, localization and etc is still something not so trivial).
Rails was born questioning the massive complexity of Java frameworks for webdev. And I still think Rails does too much stuff (like what's wrong with SQL?!), but Ember somehow found a way to have even more layers/abstractions on top of something that is even simpler than the request/response flow.
I don't like to have dozens of files with 4 lines of code extending some weird superclass in a place where I need a lot of flexibility. And MVC might have being a good idea for Smalltalk (decoupled objects, same image and different ways to deal with state) but I'm not quite sure about that for full-stack webdev (Rails) and I pretty much hate the idea for clientside only.
There's no need to think about models when all you have is json objects, this is super flexible already. How hard is to render html based on a JSON and send it back from time to time? That's why I think React's approach makes much more sense and I think most of the time you don't even need something like Flux. You can avoid 5 layers of routes, views, models, controllers, templates, adapters, etc like Ember and just go with something very very simple like this: https://gist.github.com/jeffrydegrande/020d3b3a74520c62121f .
Ember to me feels like an overengineered attempt to copy Rails approach to a world where it doesn't make much sense.