They didn't win really. There was a coalition of the "Social Democrats", Greens and Left. Because the "Social Democrats" are utterly incompetent, the state election had to be repeated because they fucked up the election process.

In the repeated election the Greens and Left had about the same votes and the "Social Democrats" lost hard. Still the coalition maintained a majority and could have continued. Instead the "Social Democrats" betrayed the coalition, claimed the election was a "clear signal against it" and became the smaller partner to the nationalist Berlin CDU, who campaigned on the notion that your surname shows whether you are a "real" German or merely a German citizen.

It is the classical liberals backstabbing progressives to embrace right/far-right politics move.

Now this current government has set out to destroy every achievement of the old government while ignoring the grave issues with housing, infrastructure, social security, climate mitigation...

Now of course after two years of that, the current government is deeply unpopular, but such is the cycle of Berlin. Have a reasonable government manage somewhat, then people elect the reactionaries, believing they would "fix" things only to fuck everything up more and have the progressive government after try to pull the cart out of the ~~dirt~~ sinkhole.