It's not exactly dead. Or else Niantic wouldn't be running so many events (monthly community days, nearly bi-weekly themed events, etc).
Forbes reports that Pokemon Go has made over $1.8B since its release. Go Fest 2018 was a rather large success compared to last year, with tickets pretty much being sold out upon release.
Your definition of "dying" is that less people are currently playing than the weeks following release day. But plenty of people do play, or else Niantic wouldn't be making as much money as they do now.
However, to answer to why the game has less players than release:
- Slow implementation of promised features. The trailer for Pokemon Go specifically showed trading, player vs player, and catching legendaries together with friends. Legendaries were implemented through raids, which was implemented about a year into the game; trading was released about a month ago; PvP hasn't been released.
- Repetitive. Niantic was not able to keep its playerbase as they got bored of the same old catching Pidgey and Rattata instead of anything exciting. Niantic has done much better at this recently with events running constantly.
- Bugs. At release, the game was extremely buggy and Niantic took a long time to fix them.
- Lack of a decent tracker. Niantic removed the well-liked 3-step tracker in favor of just showing the nearby Pokemon with no direction of where they are. They eventually changed this, however, but maybe a little too late.
- Pokemon hype. Most people started playing at release because Pokemon was involved, and Pokemon has a huge fanbase. Combined with Niantic's initial poor handling of the start of the game, people slowly left when the hype died off.