Probably for the same reason I greatly prefer cabal to stack. Stack assumes it knows better than me. Cabal just does what I tell it to do. As a domain expert, I greatly prefer the latter. It does what I want, nothing more, nothing less. Stack is a mysterious "solution" to a problem I don't have that works by doing everything differently than I do.

Stack was created because not everyone is a domain expert. A lot of people don't want to be domain experts. They just want something that works without having to know all the details. It was only able (in the business sense) to be created because so many people look at Haskell skeptically anyway, and take any excuse to back away from it. The people behind the development of stack also run a major advocacy initiative trying to get people to use Haskell, so they found it to be an important thing to build.

You don't need to try to get people to use Python. It's already broadly accepted. When people run into trouble, they just say it's the price of using Python, and aren't willing to make the exchange of giving up power to get rid of a minor inconvenience. So there's no business incentive in the Python ecosystem for making the tradeoffs stack does in the Haskell ecosystem.