It used to be a 763 in the 1990s.

It's probably not an issue of demand - you can fill a 777 between Miami and Seattle (or Boston and Seattle or New York and Seattle and so on) if you really want to - but these days it's more about frequency (there are up to 6 daily Miami-Seattle flights between AA, AS, DL and NK) and tight capacity on domestic routes.

And MIA-LAX has it's widebody back.