The reason is rather simple: there is a pretty well known and established convention inside GitHub threads to react to ideas, issues, and comments by indicating your support with a thumbs up or down.
If people who wish to show their support for something indicate this with a thumbs up reaction then:
- we need only look at one place without scrolling up or down to see how much support the idea/issue/comment has
- we reduce all the noise and pollution inside the thread so that we don't have to scroll past unhelpful "me too" comments which take up valuable screen space, especially if someone has actually posted something useful (like a workaround)
- we can more easily create, open source, and share automated tools for issue/thread/PR analysis which help maintainers figure out what the hottest topics are without writing tons of exceptions for whatever version of your "yup" is