Because if you do it for three months then:
- almost all the F2P player will give up on the game thus making IMC lose 50%-80% pplayers/potential customers
- lot of P2P players will NOT be founders because their friends will not be playing with them
- if you go check other game in the market the “early access” time is about 3 days
plus in the 1 month offer as a paying player you get a TON of nice stuff more to compensate for the two months.
It really is about making the larger amount of people happy at the same time.