They don't do freshwater because they simply don't have the knowledge or skill to pull off a nice big planted one. They can build a nice tank, but they don't have very good aquascaping skills when they can't just toss in a huge block of fake coral from what I've seen.
Tanked did some freshwater and they looked like garbage for the most part in my opinion. The beer keg one, Houdini, the fridge one and the pinball tank are the ones I remember. They did them very childish inside, quickly decorated without much thought.
A nice planted tank would take more time than they are willing to put in on the show. It would be months before the tank they planted looked robust and alive for filming. Just look how long an Iwagumi or other tanks take to really look grown in.
A big coral tank (especially since they use mostly fake stuff) usually looks nice from almost day one. Planted tanks look best a few months in.
They could do a really nice cichlid tank with rocks, ledges, stumps, but no one has likely asked for one that they wanted to film.
At least that's my opinion.