Honestly, I blame the 90s. While trying to pick wedding music we started listening to music of each decade, and the 90s had a dramatic shift of tone in rock music. I don't know if it was grunge or what that caused it, but things got a lot more dark and angry, and the "fun" type of rock music started to die out. I'm all for some good hard angry rock/metal some times, but the past 25 years or so seems to be pretty much JUST dark and angry rock with very little upbeat type of stuff. Like what was mentioned before, I think that type of rock is starting to leave the mainstream so it's not in front of everyone like classic rock used to be. That's not necessarily bad if you know where to find what you like, but it makes it tough when looking for modern music that ISN'T angry or sad.
There are a few bands out there now that are trying to bring the fun rock back, and I appreciate it a lot. I throw them out there a LOT, but I'll plug them again... The Struts are doing amazing things IMO. Their 1st album was everything we've been missing from rock since it took a dark turn in the 90s, and it's so damn fun to listen to.
https://www.youtube.com/watch?v=r61Xj0pHawE
https://www.youtube.com/watch?v=W-dbUBwqxos
https://www.youtube.com/watch?v=t63-rx0Xo8M
https://www.youtube.com/watch?v=ETiALy3pUAs
Newest single, a little slower but still solid:
https://www.youtube.com/watch?v=wXUp0g1-zXc