Because the large companies that deploy hundreds of thousands of servers need to hire lots and lots of people to make or maintain local enhancements, at all levels from the OS to libraries and utilities. In that environment it's not hard to make a local change that saves a million dollars in running-hardware costs, and it's also not hard to find Linux developers to make those changes. OpenBSD developers? Not so much. Then smaller companies do what the big ones are doing even when the potential for million-dollar enhancements isn't there, often because their founders and/or most of their personnel came from those big companies. It's the "rich get richer" effect familiar from social networks, but for software platforms.
The kernel teams at most large companies, even large tech companies, are not that big.
Sure, the pool of currently experienced Linux kernel developers is large compared to the BSDs, but if you hire a smart person (like a Linux kernel developer), and provide them with means, opportunity, and motive you'll have a BSD kernel developer soon enough.
Commercial support and network effects and costs of running multiple operating systems are more likely to be a deciding factor than developer availability. If you need to run someone else's software, it probably runs on Linux or Windows. If you run a BSD for your stuff, and Linux for theirs, that means you'll probably need more people to understand both.
If you're going to run on other people's hardware, Linux is almost always supported, although there's been some movement on BSD in clouds. Like with other niche platforms, if it's not currently supported and you want to do it, you might need to do the work -- there's a lot less community to rely on to magically make things work.
>The kernel teams at most large companies, even large tech > companies, are not that big.
It's not just about the kernel. You're talking about entire ecosystems of core utilities, filesystems, network stacks, security mechanisms, virtualization and resource-control subsystems (e.g. cgroups), performance profiling and tuning, etc. They're different systems from top to bottom, just like when I switched from 4.3 to V.2 thirty years ago.
> Commercial support
Stop right there. FAANG companies aren't going to buy support contracts. They'll hire the maintainers instead (like they did with me and hundreds of others like me at my current company). They're not going to split that effort across multiple platforms. They'll focus on one, then hire thousands of developers who don't even realize the tools and interfaces they use are Linux-specific. Then all of the wannabes will copy them, for the reasons I already mentioned. The network effect has gone way beyond any chance of reversal. Sorry.
