Almost all boards have started their leagues agreed but how many are popular? and how many have high-value international cricket players? How many are able to relocate their venue in a different country? You need money for everything and only BCCI and IPL franchises have that kind of money.
Franchise leagues are privately owned commercial enterprises and they will invest money only if they get good returns out of it. The BCCI has banned Indian players from participating in the other franchise leagues, and this has severely impacted their viewership and revenue generation from the Indian market. Only the CPL and BBL have some viewership in India, due to the presence of some of the big names in T20 cricket.
BCCI refused the permission in the past but I am not sure BCCI completely banned Indian players from overseas t20 leagues and other other cricket boards too have just kind of need if they want to participate in T20 leagues organized by other boards.