For a employment based GC, the wait time to be eligible for Citizenship is 5 years; whereas for family based GC, the wait time to be eligible for citizenship is 3 years ( if married and remained married to US citizen for 3 years). Why there is a difference in number of years? Why can not one be eligible for citizenship after obtaining GC in some "constant" number of years? In other words, why there is a discrimination in these two cases?