You asked — “Why? No one challenged anything he did as an IL Senator. And he even admitted there were issues with his citizenship during a debate with Alan Keyes and Jack Ryan.”
That’s because there wouldn’t be anything to be legally challenged in his birth certificate (as a natural-born citizen) as an Illinois Senator. But, there is something to be legally challenged, in that regard, as President of the United States (per the Constitution...).
You would be right if Obama was a ‘naturalized citizen’ while holding a senatorial office.
But, since there is no documentation to prove that, he may be an illegal alien (actually, I say enemy combatant).
THAT would be challengable as an IL senator.