For the league organizers, one of the best features of the expansion method is that they get to choose where the new teams will play from. Curating the league in that way allows them to spread it out to every corner of the country, and make the best use of cities that make commercial sense from the branding perspective.
The ISL made full use of this when it launched in 2014. They picked eight metros as home bases for the franchises, covering most of the length and width of India. This formed a sharp contrast to I-League, the top division league of the time, which followed the promotion method.
From early in the aughts, NFL and subsequently I-League saw clubs from football-heavy regions like Kolkata and Goa take over the league, vastly outnumbering the other regions. For many years the I-League did not have any representation from North India, and it undermined the league's commercial appeal as a pan-Indian competition.
Even now, ISL's inclination towards Punjab FC appears to be guided by the fact that the league doesn't have representation from North India. Whereas, ISL is extremely popular in Kerala. Kerala Blasters is the most popular club in the state and even fans of Gokulam Kerala FC regularly watch ISL. Adding Punjab FC to the league potentially opens the league up to a whole new set of viewers, and create another loyal fanbase.
Given FSDL's central push to popularize football in India, this is a compelling aspect that they are unlikely to ignore.