Well that's the thing. The Greens may vote for lefty principles in their meetings, but it's increasingly clear that the party will go down the Kretschmann route.
You have to keep in mind that there are significant regional differences - and frictions - within the Green Party. One could say that outside of ecological issues, the German Greens are de facto a coalition of two distinct political parties.
Kretschmann is not at all representative for the party as a whole... that is for the party outside of Baden-Württemberg. Only Hesse is basically Baden-Württembergish to some extent too. Together, Baden-Württemberg and Hesse make up about one quarter of the party's membership.
The NRW Greens on the other hand constitute a bastion of leftism within the party. They make up about a fifth of the party. I also can't see that the Greens "went Kretschmann" in Berlin, since they formed a coalition with the SPD and the Left here a couple of days ago.
All of this is not a very new situation, it only became a more prominent issue (and source of intra-party friction) ever since the Greens took power in Baden-Württemberg and took over the minister-president's position there.
In any case, outside of B-W Winfried Kretschmann is often - and increasingly - regarded as a nuisance. But on the other hand, you just can't get rid of him because he also the electorally most successful politician in the party. Which constitutes one of the main dilemmas the Greens are facing in everyday business.