I’m not sure what type of poly you were practicing, but I assure you that most poly people I know (including myself) very much have “long-burner” relationships. Sure, I know lots of spark chasers and the majority of the long burners have some spark chasers, too. (I do not. I am a long burner, hence why I don’t do casual dating anymore)
One does not have to be a monogamist to have relationships that they understand will lose the “spark” or NRE and settle into a nice comfortable rhythm that ebbs and flows over time. No one will ever be “the one” for me, because I’ve already had quite a few “the ones”, in mono and poly relationships. I’ve had more mono boyfriends who have thought that since “the spark” was no longer there that they needed to chase a new sparkly thing than I have had with poly boyfriends.
Also, you don’t “owe” anyone an explanation for leaving them, no matter when why or how long you’ve been together — but I do hope you have the respect to tell someone you’ve been with for more than 6 months why you are leaving.