Interests in different metal genres ebb and flow. I'll go through periods of playing more/less black metal, or death metal, or NWOBHM, and most genres rotate through. But some styles have kinda fallen by the wayside and rarely get much play anymore.
For me, the genre that has lost the most appeal over time is definitely thrash.
Thrash as a genre just didn't age well with me. As a teen in the late 80s/early 90s I loved lots of it, and my high school years were filled with stuff from Anthrax, Overkill, and Metallica to more obscure thrash acts such as Slaughterhouse, Onslaught, and Dead On. But even 10 years later thrash was sounding very dated. The goofy bands just sounded silly, the 'topical' songs were all out of date, and the music lacked the speed of modern power/speed bands, the catchiness of NWOBHM, and the aggression of modern death and black metal bands, which left thrash as this strange crossroads: it was fast but not THAT fast, heavy but not THAT heavy, etc. If anyone plays CCGs, it became the equivalent of a midrange deck strategy: instead of being the best at a single thing, thrash just did so-so at everything.
Other factors that didn't help: I had played the crap out of my old favorites, so by the early 00s I was pretty tired of hearing them. I just rarely feel the need to ever listen to albums like 'Master of Puppets' or 'Fabulous Disaster' simply b/c I've heard them so many times. Even now, when I consider playing 'Puppets', the very first thing I think is, "geez, how many times did I play that record riding the bus during high school? Maybe I should play something else that isn't carved into my eardrums ". Meanwhile, no new thrash bands really caught my attention; all the retro-thrash revivals tended to produce nothing but weak copycats. Finally, going back and listening to bands I missed the first time around just didn't pan out. For example, I missed a lot of the German thrash material. No one had any Kreator, Destruction, etc albums in my area growing up; my copy of 'Persecution Mania' was the sole artifact of German thrash in at least a 20 mile radius TMK (I also owned the only Napalm Death album in the region, for which I was a minor celebrity among the local headbangers). But when I finally checked out stuff like Tankard and Kreator, much of the material tended to be a tad too sloppy for my tastes.
Today, the Thrash shelf on my CD rack contains one of the shortest stacks of CDs, with only the classics (Puppets, Ride the Lightning, Years of Decay, Peace Sells, South of Heaven, etc) and a few sentimental favorites (Frolic Through the Park, Escape from Pain, Victims of Deception, etc) remaining.
"I'm sorry Sam, we had real chemistry. But like a monkey on the sun, our love was too hot to live"