Maybe it's corruption, but my guess is it's just never been important enough to push through. Imagine all the work that would go into phasing out coins, both politically and logistically... all for what?
It's not corruption. It's what I'll call the interest-group problem.
Pennies are an issue a few people care deeply about and most people don't. It's electorally thrifty to accomodate those few, and so electeds do. It's an easy win, particularly in a partisan environment that punishes consensus building as betrayal of one's base.
Put another way, keeping the penny won't piss anyone off enough to get one primaried. Killing the penny might.
I don't know why the US is uniquely(?) in this stasis around the denomination of money in circulation. Of course, at this point, it's pretty academic.
Pennies should have been gone decades ago. And it's still hit or miss to use anything above a $20 especially in a smaller store.
This is also true of pennies.