From the early 90's to 2011, Alternative had a clearly defined sound of "rock plus extra stuff". This was a format that had Metallica, M.I.A., and Jack Johnson on the chart in 2008, with Metallica being the highest peaking of those three odd ducklings for Alt. It changed virtually overnight in 2012, enthusiastically so, and listeners... don't seem to have adjusted to it. I've asked people my age and they generally express frustration over what Alternative radio has become and many have stopped listening or switched over to WXRT (which plays several Alt currents along with AAA standard fare). Many kids have turned to online for alt-rock because the local Alt (WKQX) won't play many modern alt-rock songs, instead playing pop.
I think it got even worse in 2018 because Billie Eilish broke out and brought sound-alikes with her. As popular as she is, Billie is an immensely polarizing figure on Alternative with many arguments over whether she qualifies still breaking out on the Internet or real life. "Therefore I Am", despite it peaking at #1 earlier this year, suffered catastrophically bad RateTheMusic scores and callouts which has led to its swift removal from many radio stations' playlists post-peak. And none of her sound-alikes like Ashe have been able to replicate Billie's success due to suffering similar backlash.
(to be fair, "Therefore I Am" has been well-received by pop radio and it also hit #1 on pop radio airplay recently, so this is specifically an Alt radio issue with her)
Interesting, Mom+Pop is sending Beach Bunny's viral pop-punk hit "Cloud 9" to pop radio later this month, right on the heels of it going to Alternative. "Cloud 9" sounds like nothing else on the pop radio format right now - it will be worth watching to see what happens to it on pop as well as Alternative.