Les Mis has to be a musical because musicals are awesome.
Musicals are god aweful...but Les Mis is awesome. If that makes any sense lol.
Although I've heard many people insist Les Mis is an opera, not a musical. But to be honest I have no idea what the technical differences are.
It's a light opera. Unlike a musical there's almost no speaking, it's sung through. But it's also not a proper opera, because it is popular music, hence the light.
A lot of people, and I'm not meaning to be critical of you here, have only seen musicals from the 40s or earlier. Let's be honest, that wasn't the best period for music in the history of the world. Few people think of modern musicals, which are often brilliant. They're thinking of shit like Show Boat and Oklahoma* as opposed to Les Mis, Miss Saigon, a lot of the stuff Steve Sondheim has written.
* Good in the day but horribly outdated and lame by modern standards. Popular music nearly always ages badly, unlike art music.