The MMR system is fricken terrible in the sense that if one is new to arena and he fks up his first 50 +- matches, he 's going to have a bloody hard time reaching 1500 smoothly, let alone 1.8k for his weapon.
Last season I remember having to work hard to get to 1.8k, and then even harder for 2.1k(*TROLL DISCLAIMER* non too great 2's setup in the counter bracket nomnom, also first time arena'er), and in S8 I started out with 2.3k MMR and basically sailed to 2.2k. Up to 1.9k- 2k rating we were losing -2 to -5 MAX while gaining like +20 every match we won.
Broken system is broken. High MMR = relative ease to get "decent" ratings. Low MMR = Good luck trying to get it up without facing counter comps and see your rating get trashed. 3's is safer in that aspect as it's rumored to be more balanced . . .