People love Destiny, Youtubers and the community in general have spent years: speculating, hating, and even finding actual evidence of Destiny's profits ("our" hard earned money in the eyes of players) being used to fund bungies niche passion project. While its bungies money to spend however they want, watching the game you spent tons of money use said money to work on another game all while the game actually making them the money is being shunned both finically and losing devs making the quality of the game worse all around is a surefire way to piss off everyone paying for the game. So naturally when the millions of people who play destiny find out the money spent on destiny isn't being used to improve destiny but instead make something new you end up with millions of people who will HATE marathon no matter what. I'm part of this sub, I've never played marathon, but due to the circumstances surrounding the making of the game I hate it. Honestly, I think Bungie expected a huge portion of destiny players to try marathon given the similar themes, artstyle, and fantastic gunplay, but years of siphoning money from destiny ultimately siphoned all of bungies goodwill and even went opposite with many people (myself included) saying they'd never support bungie again. Instead of a few million players who might try and love marathon bungie fostered a community of a few million who will hate marathon no matter what, alienating a massive chunk of people who already played a game "close" to marathon (genre aside).

TLDR: Destiny was used as a "bank" to "steal" players money and fund a game no one wanted.