No presidential candidate has ever released 30+ years of tax returns, which is what would be necessary to cover the duration of Romney's association with Bain.
But every Presidential candidate has released more than 2 years of returns. And even those two years are incomplete. McCain released 2 years during his campaign, but he had release returns every time he ran for re-election to the senate since 1986. Kerry did the same thing with his returns. And these were two extremely wealthy men.
Why didn't Obama release his birth certificate in 2008?
He did, but it wasn't enough for Trump and all you crazy people. I don't remember anyone bugging Kerry for his birth certificate or McCain. Nobody bothered Bush about his birth certificate 3 years into his presidency.