The other boards are already agreeing to a lot of unjust demands made by the BCCI. They allow their players to participate in the IPL, while the BCCI never allows the Indian players to participate in the other leagues. No major tournaments are conducted during the two-month IPL window, while none of the other leagues, such as BBL and CPL enjoy such largesse.
Truthfully speaking, IPL is another league when compared to the tournaments that you mentioned in terms of popularity etc which is why they are in the position to make such demands.
More richer the board then more influence you will have that is applied to this, and IPL mainly designed to cover the summer holidays in India where the younger generation is more compared to other countries that is how IPL got more successful and made people to engage into cricket even if they are not a cricket fan earlier due to local patriotism.