Raised this topic here https://eu.forums.blizzard.com/en/wow/t/rbg-blitz-is-a-mmr-clownfiesta/
Its known issue, mmr of blitz is so far the worst rating system implemented in WoW. At around 1.5k you’ll face a wall that you literally need to hit with your head. Some people took 20 matches, some 40, but eventually game “lets you” get out of elo hell and play higher games. Its stupid, but there’s no way around it, just keep grinding.