They mention why in the article. Data compression in markets where data is slow and expensive. As well as a small storage footprint leading to more space on their phones for other stuff (which possibly also means a lower memory footprint so it may work better as well).

Chrome and Firefox consume more memory. UC browser is faster. It can act as replacement of sports, news and Facebook app.

The question is like why those users do not use iPhone or Samsung Galaxy 7

You’re being disingenuous: people don’t use iPhones or Galaxies because they’re expensive. Browsers are free and it doesn’t cost money to change them.

And the people who author mobile Chrome all use iPhones or Galaxies or Pixels. It's about the dogfooding -- the people who write UC Browser use it on the same phones, mobile networks and websites that the users of UC Browser use, so it's written with their needs in mind.

Which is a roundabout way of saying mobile Chrome doesn’t work well on lower end devices, which is why many users switch to it. This is the reason why I (and a distant parent, I’d assume) was looking for.

I guess that's what i dont understand, what are the needs of the people in this market and what about chrome or firefox fails to meet that need? It doesn't have flexbox support, and i would imagine that would break a ton of modern websites. It isn't like these markets have their own version of html/css, everyone's on the same web, so why go for a browser with worse support for modern standards?

Users don't know/care about flexbox at all. It's developers concern.

Also UC has much more features and builtin contents than chrome. Maybe UC can provide better user experience to new internet users that think typing urls on phones to see news and videos weird. Not everyone likes minimalism.

Your last sentence is very important.

At least Chinese markets (and possibly Indian and others as well)do not seem to have the disdain for feature loading American markets do.

Weibo is one of the most popular apps in the markwt (if not most) and it’s practically an operating system of its own. I suspect it would have gotten nowhere in US markets because people would have complained about it being not focused and having a bad UI.

One less zero in the price tag looks like a pretty good reason. The article is explicitly about low end phones.