Halo is a very different type of game. It's more of an icon for the platform, and more family friendly. It's new more approachable and "friendly" art style is a huge shift or drawing old fans back with something that is recognizably Halo to get them to at least check it out, and it's friendly enough that like Fortnite it won't alienate anyone for being this hardcore shooter.
Gears has it's niche with being an ultra violent cover shooter and it's fanbase. A lot of what we see in Gears 5 is focused around accessibility and making sure people that try it don't put it down shortly thereafter due to frustration with it. It's also fundamentally "unfriendly" to pockets of audiences, given it's language and extreme gore. It'll never appeal to 13 year old timmy's mom as appropriate to play in the living room (I hope). Gears also has some huge followings in places like Mexico and South America, where the markets and demographics I imagine are a bit different than what we have in the US. Gears 4 is still like the #3 most played game in Mexico, with I think Fornite and GTA above it.
