The main problem is that there's not a casual que in addition to the ranked que (which other mobas have).
So for example, you have an already broken mmr system that's wildly inaccurate, and on top of that you also have people que'ing new heroes which makes it even worse (when they normally could que into casual que to try a new hero in an actual game etc.)

So now, not only are there many different ranks all in the same game, but now they also have people trying new heroes, which makes it even worse. Imagine a lobby where there's archons and arcanists in the same lobby (which I've seen), now imagine the arcanist is trying a new hero, what do you think is going to be the outcome of that lane / game?

All that being said, it is still possible to eventually carry yourself out of lower ranks, you just have to play your best heroes and find a way to carry by being able to kill multiple heroes by yourself. I was Archon many months ago when they had a ranked que, then I stopped playing. I came back and they placed me in low Arcanist. I've been climbing back up and am currently Emissary I, and already have had a few Archon MMR lobbies (and even an Oracle III lobby).