Here's a good article. https://danluu.com/monorepo/ It cites simplified organization, simplified dependencies, superior tooling, and easier cross-project changes.
https://blog.ffwll.ch/2017/08/github-why-cant-host-the-kerne... is another good one. You can build issue/PR tools treating a monotree as containing multiple repositories; that's how Google and Facebook do their work (including checked-in OWNERS/MAINTAINERS files).