Brendan Eich IIRC said they looked into building Brave on top of Webkit but it was so hard to compile and embed across all three platforms that they went with Chromium. Same story with Gecko.
So that's another reason why we now have a Blink monoculture: because the alternative engines didn't spend any effort in making them usable by third party applications.
WebKit is used by the WebkitGtk library which all of these include.
But it's been stagnating for years and it's still not a viable replacement for Gecko and Chromium (broken scrolling on scaled screens, WebAuthn is unsupported so I can't login to my email account, etc.)
In any case, if I recall correctly Eich's comment, the biggest difficulty was building webkit for Windows. The documentation was very out of dated, and it's understandable given Apple's focus on its OS. Even this GitHub says it's a web engine for "macOS, iOS and Linux".
Brendan Eich IIRC said they looked into building Brave on top of Webkit but it was so hard to compile and embed across all three platforms that they went with Chromium. Same story with Gecko.
So that's another reason why we now have a Blink monoculture: because the alternative engines didn't spend any effort in making them usable by third party applications.