Romney was pressured into releasing his tax returns - something that has no bearing on how he would run the country.
After 4 years we still haven't seen Obama's transcripts.
What do you think his transcripts would look like? The guy gained admission to Harvard Law School (A top 3 law school) and ended up as president of the law review, which as far as I know is an honor reserved for someone with a very high law school GPA. Also, as far as I know, law school papers and exams are graded anonymously with students' exams and papers only being identified to their professors by a code.
I know he probably benefited from affirmative action, but still, Harvard won't admit just anyone even if they are black. So, I suspect that you'd see transcripts full of A's with a couple B's here and there.
He also worked a job at a large national law firm, and they are very selective even if you're black. He also worked as a law professor at the University of Chicago law school, which is generally regarded as the nation's 6th most prestigious law school.
If you're trying to prove that he possesses merely average or just good (as opposed to excellent) academic ability, you're going to fail epically. Maybe he should release his transcripts just to shut birther types like you up, but he probably hasn't received enough requests for it to even be on his radar.