Also because the mainstream manufacturers don't want to have to support Linux.

There is less hardware support for Linux than Windows on laptops -- largely because very cheaply made components just have their firmware loaded into them by the OS when it starts, and since they're largely proprietary firmware they conflict with open source licenses.

Linux laptops are just flat out more expensive to make, because you have to use more expensive components that don't do that, confirm compatibility, and have everything setup before you ship it. Also manufacturers don't preinstall bloatware because they feel like it. It's because they get paid. The kickbacks for preinstalling bloatwave well exceeds the cost of the Windows license.

So preinstalling Linux is more expensive component wise, support wise, and bloatware wise. There's little reason for companies to do it, unless they're trying to court software developers. Dell and Lenovo and others court software developers quite well. But there's little incentive for them to try to increase Linux's market share.