Thats because you:
A.got all ship upgrades.
B.most/everyone's loyalty. (the minimum is 4, but choices have to be on the mark.)
C.Went straight in after you know what.
D.Made Smart decisions for squads and specialists.
E.The unknown variable when people 'hold the line' is good enough to hold whilst you kick some reaper butt. (when the 'defence' is greater then the 'attack', no one dies. if its lower, someone dies and if its TOO low, they all die. in order from unloyal to loyal. some people are better then this at others... and loyalty matters here too.)
and chakwas always lives, except if she doesn't get an escort back to the normandy.