Originally Posted by
MAH4546
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.
There are only two flights between SEA and MIA. Both on AA. There are 4 more to FLL (NK and AS). So it isn't a frequency vs capacity thing - there just isn't that much demand for this flight.
I'm sure you can fill a 777.. but it won't be worth the airlines while to do so.