This is a question that's been asked for years since the last time Guitar Hero was relevant & since Rock Band first stopped updating their DLCs. What does everyone think?
For me, there's a few answers honestly, let's go through them.
1. Poor sales. An obvious one, but poor sales really did them in, nobody bought their titles besides hardcore fans (by the end of it). I certainly have converted a few people towards these game series'. Got them all hooked on these games like never before.
2. Price, The price can be contributed to the lack of support, I mean, between the peripherals breaking & stuff like that, games themselves were fine, unless we're talkin' Guitar Hero then you were buying a new game one after another (if you were dedicated like that), for Rock Band really you were only buying DLC for 1.99 a piece & the occasional pack or album which were decent prices in itself. Cost does come into this for sure.
3. Rocksmith/Clone Hero. I've been hearing that Rocksmith was a huge reason for the downfall of interest in these rhythm games, & while I somewhat agree, I think Clone Hero took over now more than Rocksmith has. Sure Rocksmith is for those who want to learn guitar for real, but Clone Hero is for those who want to play but don't want to be limited besides the fact it took the streaming/twitch community by storm.
Right now, I think that for me one reason why I don't play Rock Band 4 is not because it's a bad game or because DLC is unreasonable, but because Rock Band 3 is reliable, & customs really can't be beat. I think with everything found on this website alone it simply is the best way to play Rock Band. Sure, we went out & got Rock Band 4 & instruments, some DLC & the expansion. I honestly found myself going back to RB3 more than I was playing RB4. If RB4 had Custom track compatibility, then I really would switch over at that point, but because they patched it completely I think i'll just stick to RB3