On my commute in to the south side today I contemplated this question. I do think some very valid points have been touched on here.
1. Yes, pop has reigned supreme for about 20 years at this point.
2. Yes, I'm sure we had it WAY easier as teens in the pre-social media era. 1996 and 2018 are absolute apples and oranges.
3. Yes, downloading did play a sizable role in killing things.
4. Yes, some radio stations have changed formats (although this is more of a byproduct of rock no longer being on top, however it can even be argued that mainstream commercial fm radio is no longer even that relevant anymore anyway)
5. Yes, I agree with the comment made about many contemporary artists crafting songs that are bland and generic bar rock, back ground noise that no one remembers and hour after hearing it
However, here's what I can add....
Starting with number 5 as referenced above; people (especially kids) today don't have the attention spans they did years ago. This accounts for two points I'd like to add. One is, these short attention spans are exactly why all of the music today is crafted to be super loud and in your face. Something "big" is always happening in a lot of these songs because it catches attention. Secondly, aside from everyone's issue of expecting instant gratification translating to short attention spans, this also contributes directly to a large percentage of people who don't have it in them to sit through a whole album. The album is the holy grail of rock music.
In modern times it's all about singles, both downloading and streaming. Without albums, what bands are truly building legendary legacies? It seems like today it's all about having a hot singl, you get your fifteen minutes of fame, then you fade away. Can anyone here name even one contemporary band/artist that is selling big numbers of singles to whom people are really latching onto and investing themselves into heavily? A band/artist who is on such a path as to where they/he/she will be seen as legendary in the future?
Look at all the big summer rock tours... All of them are artists who are well past their prime and/or considered the elder statesmen of rock. I don't think there are any cutting edge bands who have a huge summer tour that kids are excited to see in mass numbers. I just don't think that exists like it did in the 70'/80's/90's. I think these package deal destination festivals (i.e.: Lollapalooza) are what kids go to if they're going to see rock performed on a big stage.
Rock n' roll is a young person's game and I think a huge part of the reason rock isn't on top is because perhaps kids didn't ever have the same chance to get into it growing up as I did. Throwing the whole internet and social media thing aside for a second.... I'll tell you how I got into hard rock was first, discovering a radio station (which does go back to point 4 above). Without proper rock radio stations, kids aren't being as exposed to it. Though there is plenty of it out there on YouTube etc. they dont know any better to go looking for it on their own because they aren't being heavily exposed to it due to perhaps the lack of a good rock radio station as many have changed formats. I flipped on the radio the other day to the only station that considers itself "modern alternative" in Chicago. While I think I recall hearing one 90's rock song during the time it was on, I heard a lot of what is considered "modern alternative" and let me tell you, it sounded way more like pop to me. It certainly wasn't guitar driven music. Sounded more like a slightly louder/more in your face version of music Pink was putting out 16 years ago to me.
Another reason kids aren't getting into it is the death of the all-ages show. Rock shows were places kids went to smoke cigarettes, sneak a beer, mosh, and act crazy; just being a normal rebellious teen. As a kid I used to be able to go to licensed businesses to see up and coming local bands and nationally touring relatively unknown to semi-known acts. Fireside Bowl was a huge deal in Chicago. A handful of acts that played there became national scene players on the big stage eventually. Those bands didn't start huge, the kids got to watch the seeds get planted, they got to invest their time and passion into the bands and watch them grow. What isn't talked about much is that 9/11 marked the beginning of the death of the all ages venues due to rising insurance costs. I can argue all day how this killed rock because of a simple point. To kill a tree, you have to kill it a its roots. The kids always have been and always will be the roots of rock. Take them away and the tree will die.
Another point that should be raised is in regards to the internet, but doesn't revolve around downloading. The net is the biggest blessing for music and the biggest curse at the same time. It's a blessing for distribution in the sense that I can click a button and someone in China can have my music. However it's also the biggest curse on rock in ways beyond illegal downloading. Remember Myspace??? They had the single best filtering tools I've ever seen. You could find any band of any style in any region that you wanted to. You could easily put together your own D.I.Y tour just by using the tools that site presented you with. However, holy fuck, did MySpace ever help ruin rock at the same time. Everybody and their mother had a fuckin band and would solicit you. Everywhere you turned someone was saying "hey we see you like bands x,y,and z; so check us out!" It totally ruined a major part of rock for me personally which is.... I used to always look forward to seeing opening bands at shows. It was exciting because I just MIGHT stumble upon something good, new, and exciting. However in a post MySpace world I have lost all interest in seeing opening bands at shows. Everyone soliciting me absolutely ruined it for me. I'm sure others out there share my sentiments on this.
The final point I will make this morning is.... what made rock special is it possessed an element of danger. There was certainly an "anything can happen" vibe about the shows, the characters involved, just the whole experience. That element of danger is gone and has been missing for quite some time. It's been neutered, sterilized, and wrapped up in a cute little package that reads "J.P. Morgan Chase Bank Tested: Suburban Mom Approved". People easily pick up on shit that isn't genuine. It won't sell.
EDM shows are where the danger is these days. EDM shows are where anything can happen. If you're looking for teens/young adults in mass numbers you'll find them there. That's where the action is. It's not my thing and time has certainly passed me by.