Kendrick and Billie may be huge artists, but their music is forward-thinking and "alternative" in a lot of ways. I mean, a quick look at 1996 shows that Metallica and Soundgarden headlined that year, both of which had massive number 1 albums at that point, so it's not like they've ever been averse to booking mainstream headliners.

I think the issue is people are looking at "alternative" and thinking "alternative rock." New alt rock bands aren't headlining nowadays because rock isn't as popular. But Billie and Kendrick are great examples of hugely popular stars that's still doing something alternative, quite frankly a lot more alternative than most of the cookie-cutter alt rock bands of the 90s. It's just that rap and pop artists don't tend to get credit for being forward-thinking in the same way rock acts do.