What caused the collapse of music games?
Oversaturation of the market, of course. The gameplay has barely changed since the first guitar hero, and for a good reason: the only reason people play these games is because of the songs. So as soon as there was DLC (since rock band 2, iirc), these games had barely anything to offer that the audience wanted.
Both companies obviously knew this, and sort of raced each other to bring out song packs (games, if you will) until things weren't profitable anymore.
Still...a crowdfunding is pretty pathetic. Despite the milking process, these games still made millions (
ahem). So now they want 1,5 million dollars to allow people to buy something that is essentially a virtual music shop? Yeah...no. If I want to guitar hero/rock band on pc, I'll go with frets on fire. If I want to play music, I'll go with rocksmith.
(it's a guess, but I think those managers at harmonix know this as well...but are too incompetent to calculate whether their game will make a profit on PC regardless).