If one is marketing to an audience of North American visitors, warmer winter temperatures and less effect from cold fronts reaching the island is probably a bonus.
The modern more adventurous tourist.....and often those with more money to spend.....wants warm weather, minimal humidity and plenty to do on their short vacation.
Many of the hotels shown in the video are located in the Baoruco area which does get it's rainfall as the lush green tropical canopy confirms.
Variation from a flat landscape and a good beach with plenty of eco options will appeal to the target market this area is after. And the development money will, or rather does, like the interest that can be attracted from that lucrative market.
The market you refer to is tiny. Even eco-tourism is tiny.
And the SW isn't just a little warmer. It can be brutally
HOT. Like an oven. Seriously. There are a few oasis but they are still really hot...they just have some water.
It's an interesting area no doubt, but after a few days there we're happy to leave for cooler climes.
Additionally, with the exception of Bahia de Las Aguilas, the beaches are suboptimal for tourism. Mainly rocks.
People come to the DR looking for a tropical Caribbean fantasy. The SW ain't it.