Probably because he felt it insulted his intelligence.
My own reaction to those who raised the non-issue, disprovable by the passport issuance, was Fuck 'em, it's bad faith racially bigoted bullshit.
Why didn't it dog him in his state legislative career, his stint in a law firm, his job as a Constitutional Law instructor at the U. Of Chg. and in his successful run for the U.S Senate?
Law of averages, somewhere along the line his citizenship was part of a background check.
Birthers ignored all of the above, and common f'in sense as well
You always knew.....shit. Yours is a knee jerk reaction that overlooks all of the facts I've laid out