I wonder why this term came about? Perhaps in that brief interval when there were 49 states: Alaska(upper/most northern) and the other 48(all lower/southern to Alaska).
With the addition of Hawaii, there are now 50 states: Alaska(upper/most northern), Hawaii(lowest/most southern), and 48 in the middle. Yet we continue to call them the "lower" 48.
"Contiguous" is more accurate than lower, although there are plenty of non-contiguous land portions. I don't like "continental" either, since Alaska is part of the continent, too.
I guess "middle 48" just doesn't sound catchy enough! Maybe "eastern" 48 works, aside from the portions of Alaska in the eastern hemisphere.