Linux

What?

Because it's free. That's the beauty of FOSS software. But only is it user freedom respecting but also often free of charge.

In third world countries like India where only a small percentage can afford a Windows licence, Linux can thrive. And once people see it can do anything that Windows can, and far better with no security concerns, plus they can modify and build upon Linux, they tend to adopt it with passion.

If you're starting a business in India and have little money but need an OS to run your business, Linux can do that for you for free.

Government departments are slowly ditching windows because everything they need to do can be done on Linux without malware attacks and every application screaming "buy me!" every time you open it.

Also educational institutions (at least in my State) do give basic Ubuntu training. I learned Gimp ,Inkscape and Libre office as part of my curriculum.

And we love free stuff.

I suspect it is a combination of its being free, working well on older hardware, and the tech literacy in India.

Software development and engineering are important aspects of the Indian economy. Linux is arguably the best platform for that kind of work, especially in the cloud. Tech support of those kinds of systems require the same skills.

Given how well Linux runs on older machines, I consider low Linux penetration a hallmark of rich countries.

In my own household, Linux goes on all the older hardware ( including Macs ). That has really extended the length of time before hardware needs to be replaced. It also means that, over time, the percentage of active equipment using Linux has increased.