Also, and maybe most important:
In the majority(?) of cases it did not work.
Landlords found absurd ways to work around it. Many not necessary fully legal but only in a way that if no one who is "hurt" by it sues the state can not act.
There where enough people which where open to pay the de-facto increased "not so legal" Rent, and as such would definitely not sure.
A common trick was to pair the rent contract with Renting some token furniture for completely unreasonable (and as such somewhat illegal/moon-) prices. (note: this wasn't the same as the rent prices for furnished apartments exception thingy mentioned by others)
Again if both renter and rentee are happy with it there is nothing which the state could have done.
Also more apartments where sold instead of rented out, in turn decreasing the amount of rente-able apartments while the demand did change (maybe except due to COVID, I would have to look it up). Furthermore it's not uncommon that such apartments do not enter the rent pool again even if the person in question leaves Berlin (and then illegally perma rents it over RBnB to tourists, or keeps it empty or resells it or ...)
Lastly due to majorly increased renovation costs (as far as I know) and now rent caps it was basically much less likely for any new apartments to be added to the pool. And even (potentially necessary) renovations of existing ones was much less likely done (as far as I have heard from people involved in that business, through not the very big players, but it matches the experience other cities which did impose rent caps had).
And just to be clear Rents in Berlin reaching a point where it's problematic for many people from Berlin to pay them IS a massive problem. Just that this law as far as I can tell would not have solved it and maybe even made it worse. Also yes Rents had been unusual low in the past (I'm personally profiting from this) but now they have gone beyond any reasonable price in many cases (at least in context what many people earn in Berlin).