Because its technically not a real cap as a hard barrier you can’t pass, but a drastic nerfed mmr coefficient which results in the same effect → artificially reduced mmr!
And since you are a fan of looking out for official announcements, don’t oversee this one:
Which is virtually proof that they are artificially screwing around with the MMR system! If you want to know what happens to MMR without them messing around with it, then just take a look at the BBG ladder and compare it with the shuffle ladder. 