Those leagues were filled with guys who just needed some "developing" and their dreams were to make it to the NFL and 99% of them failed to make it there. The league failed because of a combo platter of that the product wasn't all that good and people didn't want to watch it.
The same thing would happen here. Nobody would give a shit about yet another football league. There's big signs of football fatigue already. Fans are complaining about the NFL on Thursday night as it is, and that's with the actual NFL and not a bunch of bottom of the roster, easily replaced scrubs. Who is going to pay to watch bad football? (insert Bears season ticket holders joke here).
The NFL is driven solely by money and a D-League is complete money pit, even if it's just in the US. No way the NFL is going to start something up that will only lose them money.
Besides, so long as the NCAA is giving the milk away for free, why invest in another herd of cows?