It's 14 or so years old and people still play it religiously. Smash 64 is still a similar price for a similar reason.
Edit: if you're not hell bent on getting Melee, check out Project M. It's a mod of Brawl made to feel like Melee. It'll be cheaper to get a Brawl disc if you have a Wii than it will be to find a Melee disc for your GameCube. I like it because it has a lot more characters than Melee (it even adds Mewtwo and Roy from Melee back in). It also adds some new gamemodes, new costumes, new/better stages, and reworks a lot of Brawl characters' movesets.
I used to have a Wii and Brawl years ago, but recently got a Gamecube ([i]way[/i] cheaper than buying a Wii again) and have been wanting Melee for a while. Literally everyone I've seen/heard talk about Melee says it's the best in the series, so I'd say I'm quite hell bent on getting the game. Everything I've heard says to me that it's quite worth the asking prices, but I am still baffled used copies costs on average more than a brand new $60 game.