There is a reason why he did not. He is a selfish egomaniac. It never occurred to him that this dirty laundry needed to be figured out before running. Well, anyway, this is the least of his problems.
I believe the anti-Newt argument is that he didn’t care about his baggage, because he never expected to be a factor in the race, he was just running to help his book sales and to make him relevant again. Also to burnish some conservative credentials because he saw that the real speaking money was in the tea party movement now.
How was he to know that all the conservatives would either implode, be insufficiently flashy at the debates, or be hopeless bores?