In my opinion, the reason why dtg printing on dark polyester does not work is the hand that results from pretreatment and multiple layers of ink on the polyester (which is usually very soft). It alters the customer's expectation of how the shirt should feel.
I have heard of several tricks (flash curing the shirt longer, opening the press more,...) to curing CMYK ink on polyester and none of them are that great for production runs. The curing of the white ink can be challenging to do on cotton to get a good wash test, it is even harder to do on polyester. The large amount of technical support that would arise if the manufacturers and distributors start promoting the printing on polyester is probably not worth it.
Just my opinion,
Mark