Because no one high up in government wanted to challenge his citizenship and specifically whether he was a natural born citizen for fear of being publicly smeared as a racist and the very real possibility of loss of status, position and employment. The smearing is done by powerful Democrats as their way of retaining power.
The smearing was also done by Republicans that had their own stable of ineligibles they wanted to run.
Jindal, Rubio, Cruz, Haley and most importantly, George P. Bush.