Because:
- its a fighting game that can be tailored to both casual and pro players.
- It has a large roster of characters, most that would generate a sense of nostalgia in many people.
- It encourages local play with friends and family (3DS obviously excluded here but not the Wii-u version)
- It is one of the very few decent 4 player fighting games
- It has ALOT of items that keep it fresh each round, you never know how the match might play out
- Its funny! Being knocked out by a green shell or slipping on a banana peel is more tolerable than getting beat with hadouken spam.
- As you say, the music is great and usually remixs and original songs from classic games (More nostalgia)
- For a fightning game at its heart, it has alot of content. Map maker, several single player modes, online play, custom rules ect ect
If you're not into fighting games at all then fair enough, you probably dont get the hype. If you do like fighting games then there WILL be something here you like, it does everything well enough to have something for everyone. I myself love the games and iv had tons of fun playing with my friends / family casually. I fully get the hype and im a part of it, so many characters to pick from this year too. Hopefully the content on wii-u is as packed as it was in brawl and if it is, ill probably be still playing it when the next one comes out.