Why is 'unexpectedly' common but 'expectedly' not?
Is there a rationale for why any word is common or uncommon? Common, in this context, simply means it is used more often. But certainly there are more drastic instances of where "un" or "dis" words appear with little to no use of the un-prefixed version. Example: disheveled but no sheveled. Unkempt but not kempt. Inscrutable but no scrutable.
"Lonely negatives" some call them [http://wordsnooper.com/2011/02/24/scrutable-inscrutable/]