Because Islam is more than a religion, it's a political structure as well. Christians founded this country, and believed in religious freedom, Islam would not have done the same. You have to believe that there is no difference between Islam and Christianity to ask your question to begin with.
What the hezzli shizzle ****? You don't have the first damn clue what you are talking about. All mainstream religions are political structures. Can you name a single non-Christian religious Federally and State recognized holiday in the US? What the hell is going on? How can people make statements that are so unbelievably out of whack with Politics 101?
So the fact that recognized religious holidays in a nation that is 95% or so Christian are also Christian makes Christianity a political structure? Are you really this big a dumbfuck? What holidays would you EXPECT the government to close its offices and give its employees off? Jewish, which comprises maybe 1-3% of the population?
I know it just galls you and your cohorts, but this nation was founded by people from Christian denominations and remains culturally Christian. The fact that the government recognizes this about its constituents doesn't exactly equate to making Christianity political.