@Kobald
Dun actually know. As far as I can tell it's just that sentai just lost a lot of steam in most places.
W.A.G.'ing, I'd say it's cause the genre was made primarily to sell figurines and Evil Org troupe tied heavily with Sentai shows.
For more Wild Ass Guessing..
As better tech to make higher fidelity figures for adults to collect and more games and better games / stuff to play with for kids (from Yu-Gi-Oh to CoD4 to Kingdom Hearts) the less attention there was on Sentai products, and like I pointed earlier the genre was practically designed to sell toys. So less funding for Sentai (and Evil Orgs) as more lucrative investment options opened up in other directions. Less funding = less shows = enthusiasm .. etc...