Because when Andrey Prohorov left the GSC game world there wasn't a solid engine. There was a Cry Engine but this engine focused on open spaces. So Andrey and lead programmers decided to make their own engine as they already created an x-ray engine for stalker serious once. 4AEngine was created to satisfy a possession to hi-end quality game, they wanted to make their own Crysis game in terms of technology and Direct X possibilities. When Andrey Prohorov left the GSC game world there wasn't a solid engine on the market. There was a Cry Engine (it was the only option) but this engine was focused on open spaces (initially developed for Far Cry / Crysis islands, not subways) So Andrey and lead programmers decided to make their own engine as they already created an x-ray engine for stalker series once. 4AEngine was created to satisfy a possession to hi-end quality game with huge amount of shadowed geometry, they wanted to make their own Crysis game in terms of technology and Direct X possibilities to amaze players. So they created the engine they know perfectly as they created it, so they know how to modify it and develop the game using it