I remember this trend started as a way to feel good about yourself for buying a bottle that was not made of plastic

. Kleen Kanteen is or was the biggest brand.
Some people use metal bottles because they're worried about microplastics and xenoestrogens or whatever leaching from plastic water bottles into the water. Which is futile because that shit's already in the water supply and can't be effectively filtered out.
I think those are the two biggest factors that drive the metal bottle trend. That being said I have one and don't hate it. I only really bother to use it for camping/hiking tho