A lot of factors, really. The first and most obvious one is age. Every single person who enjoyed Old Halo and met or started here in those days are all now fully grown adults, with either careers, significant others, families, major responsibilities, or all of the above. Or dead--unfortunately. The second biggest reason is moving on. In addition to growing up, our tastes and desires have changed, too. Everyone has moved on to more advanced, bigger, current, and--no point in denying it either--frankly better games. While classic games may always be in our hearts and while we may still own them or play them, inevitably we find ourselves gravitating towards the newer stuff regardless. Even video games themselves are taking a backseat to other forms of entertainment. Hell, I just binged the entire series of Bojack Horseman over the course of 10 days, and I'm 1/3rd through The Ballad of Buster Scruggs. I barely even play video games anymore.
While FPS's are always going to be here to stay, as of right now, they're not the most popular gaming genre out there--battle royales are. At least for this month. So video game genres might be a semi-permanent fixture, fads will keep them rotating as flavor of the month. And right now, FPS games aren't up top.
Finally, calling Halo "dying" isn't entirely the most accurate term to use. While I don't know exact numbers or semantics, tournaments and eSports are still featuring the New Halo games on an almost constant basis. Halo is essentially adapting for a new generation, and going in a different direction because of it. It may not be a direction that Old Halo players would like, but so many new changes have been made to reflect the newest staples of first person shooters.
To paraphrase and steal from an old friend, I'm going to drop a truth bomb that you guys won't like, but only because you'll realize deep down it's true:
You may not want to admit it, but you all don't really want the old Halos back. You want to be 14 years old again, gaming in your best friend's basement subsisting on a diet exclusively of pizzas and chips and orange soda. Bad news. You're not the core demo anymore. If 343i took out Sprint and
brought back the 3-shot pistol, you'd still be working 9 hours a day and playing Overwatch or Apex Legends for the only half hour a night you've got free.
It hurts to say it, including myself, but that's the fucked up hardship of it: Halo isn't dying.