TL;DR AT THE BOTTOM
Let me first mention that I do have a bias against the franchise, and I recognize that. I would like to enjoy the series, but these things keep me from it. I'm going to try to keep it as factual as possible instead of using "in my opinion", "I think", etc. Here's a bit of a synopsis on what the game was vs. what it has become.
The Call of Duty series' multiplayer has, until the early 2010s, always been a casual FPS with loadouts, perks, killstreaks, and level-based progression. The maps were small-medium sized for the most part, and the matches were fast paced. It's a simple formula and it worked well for a long time. It's what defined the series and is what myself and many others remember it best for. Modern Warfare 1 and 2, World at War, and Call of Duty 3 are fantastic examples. They weren't needlessly complex and had no lootboxes or season passes, yet the earlier games are regarded very highly among fans when compared to the likes of Black Ops 3, Ghosts, Advanced Warfare, etc. The only microtransactions were map packs, not useless cosmetics or loot boxes which may-or-may-not give you a legendary item, thus a competitive edge against other players who didn't spend an additional $20. Modern Call of Duty games are simply weighted toward making Activision as large of a profit as possible at the expense of delivering sub-par quality games. They instead rely on gimmicks instead of creating memorable maps, weapons, or gameplay that flows well as they have previously.
I digress, though. So, battle royale in the Call of Duty series.
The problem is that there's no inspiration. Anyone can include a battle royale gametype and bank off of its guaranteed success, which is obviously what's happening in this case. They know it's going to attract Fortnite/PUBG players and sales will inevitably skyrocket. This may not be the case in one or two years, however, when battle royale may be replaced by another trend.
I'm not saying it's wrong to experiment with new gamemodes and take risks for something your players may or may not like, but there's no risk at all in essentially porting another company's game into your own. Companies need to make money, yes, and following trends is a good way to guarantee sales, though a lazy one. The problem is that when companies do this for multiple years in a row, we somehow wonder why the entire gaming market feels stale and saturated with nearly indistinguishable games. The cycle continues.
It just so happens that battle royale is the flavor-of-the-month genre right now. It's flooded the market with clones and knock-offs, usually of abhorrent quality in a rushed attempt to make a profit before everyone's sick of it. What I'm saying is that Activision could have spent their development time creating a single player campaign with good writing or more maps for multiplayer, but instead, they're just milking the cash cow at the expense of originality. Battle royale is a modern day gold rush for developers, nothing more.
Oddly, I want to like the Call of Duty series, which is why I'm upset in the first place. I loved the older games, and for a while, it was my go-to FPS on consoles. Every game felt different, whereas if you showed me a screenshot of any modern Call of Duty, I would likely have to guess if it were Black Ops 3, Advanced Warfare, or Infinite Warfare. Now, it's all about high-tech aesthetics, wall running, jetpacks, and essentially trying as hard as possible to be Titanfall. From what I've heard they removed jetpacks which is a step in the right direction. Giving a player so many locomotion options is overwhelming, and removes the simplicity which made it an appealing FPS in the first place.
Battle royale doesn't belong in the Call of Duty series, nor do any of the overly complicated mechanics or additions it has recieved in the last decade. It used to have a formula that worked, and people loved. They found a formula that makes a larger profit, and are likely going to stick to it unless we speak with our money by not buying uninspired games.
Sorry for the wall of text.
TL;DR: By being okay with Activision cashing in on the battle royale trend, we as consumers are being complacent with companies producing mediocre games that rely on trends to be successful instead of creating memorable experiences like they used to.