A lot of these large volume OEMs depend on extra revenue from the Windows bloatware that they preload (antivirus, games, social apps etc.)... Usually they get a kickback based on volume sold.

This stuff largely doesn't exist for Linux, so they're missing out on this profit for any Linux preloads. It's also why you see Linux preloads mostly done by more boutique PC builders -- they've already charged a reasonable premium on the build.