It's cheaper to manufacture 16:9 panels across the board, meaning cheaper screens for us, so even though it's an inferior aspect ratio, a lot of people settle for it because it's better value for them.
16:9
is best for TV, though. So... if you're mostly watching lots of TV on your monitor instead of gaming or productivity etc, then it's cheaper AND better.
There are still a few 16:10 screens out there - plenty of 30" 1600P screens if you look around (albeit at 60Hz and without Freesync/G-Sync), and a friend just picked up a Dell U2415 last month. I'm surprised so many gamers are settling for 16:9 instead of demanding better - usually we're up in arms over the smallest things! I wonder if many people simply haven't tried 16:10 and so they don't know.