#1 The heat of Derby

This is the biggest factor that no other football clubs in the country can bring to the football grounds of ISL - 'The Big Derby'. Being a part of FIFA's Classic Derby List, ISL will only have benefits on this factor.
For a new and young league like ISL, to have a century-old history would mean a big thing. To host this match would be a great occasion for the league. The two clubs too won't be affected as the ISL fixtures let every team face each other twice a season, thus maintaining the number of times the two sides lock horns.
In a league that follows semi-finals, a knockout match between these two teams will make things interesting.
Having the emotional attachments of football pundits of Kolkata, a major share of oppositions to this cash-rich league can be limited by this move. Thus, more than the clubs waiting to make this move, the league holders will be the primary benefiter of East Bengal and Mohun Bagan entering the Indian Super League.