Because of many many reasons
1- not too many care about it like I do.
2- too much cricket
3- not much success even at Asian level to boost its popularity
4 - very poor facilities in most cities
5- inspite of a rich history of club football (especially in Bengal/Goa) nobody watches domestic football on tv, they would rather see Premier League
6- No money in the sport

Like in every thing you can think of, India has the talent
What we lack is the motivation, physical fitness, mental strength, financial backing and organization (read sponsors) and the real passion for this game.