I'm surprised AA or DL haven't rolled the dice with a seasonal B752 on MAN-BOS just to test the waters. I would have thought there was easily a year round market even on a narrowbody at least.
AA have operated MAN-BOS in the past with a 757. I don't know the reasons why it was discontinued, but with the economy improving maybe it's time for an airline to try again.
As for AA on MIA, a seasonal B763? Maybe, but perhaps they feel the MAN-Florida market is already flooded?
The only bit of the Florida market that's flooded with capacity is to MCO/SFB. We drove from Orlando to Miami in September and that's a 3 1/2 hour drive - something that most people would rather fly instead and it's understandable.
Also, Miami is a different sort of market to Orlando as it's a major city/destination in its own right (especially for the under-30s heading to Miami Beach), there's a cruise terminal, it's an AA hub for Caribbean/Latin America flights and a gateway to South Florida/Florida Keys. Orlando is obviously more family and entertainment-orientated, as well as being in the middle for those looking to holiday on the Gulf Coast or Atlantic Coast.
Some of these reasons are probably why Thomas Cook felt confident in launching MIA flights alongside its MCO flights from MAN and why BA and VS serve MIA from LHR in addition to MCO from LGW. Don't forget, BA also fly to TPA from LGW which can also act as an alternative to MCO for those heading to Orlando as it's only about 90mins drive along I-4.