You did read the discussion right? Aside from simply realizing the rp is not what they thought it'd be (which may lead to negative votes from upset GMs), rpers may not get along with others in the rp (which may lead to negative votes from upset rpers), may lose Internet due to an outage or being grounded (which may lead to negative votes despite having a valid reason), may feel forced to disclose what feels like completely personal problems such as depression to relative strangers in order to avoid negative votes, or may just lie altogether to avoid negative votes. And sometimes they just forget to go on the Guild because something big's going on offline like an upcoming exam or work, and no one in the rp has non-Guild contact information. In short, people flake for a lot of reasons beyond 'let's screw over your rp' and these reasons can lead to system abuse.

Also, I do note you bring up Youtube. The thing is, Youtube also has a larger population. People with Google+ OR Youtube accounts can vote on a video. A downvote doesn't mean much because there are millions of others who can provide an upvote. At the time of this writing, the Guild has less than 60,000 accounts registered - some of which are inactive, some of which only post in non-rp boards exclusively. As such, the actual number of potential voters is much smaller. If the ability to vote depends on rping with these people, then one negative vote can be devastating.

Finally, at this point, 'participation' still hasn't been defined. Are they no longer an active participant if they flake after ten posts after they post a CS? What about 20? What if they flake a year later? 4? Never - and let me say, as a long-time rper, it can easily take 4+ years to reach the end of a rp with a decent plot. Me, I was in a rp that lasted 3 months. We didn't reach anywhere near an endpoint, and those who vanished didn't leave an explanation why. Do I say that, because they only lasted 3 months, they're unreliable? It was a lot of work lost, after all. Or do those who chronically skip out after a month or so get skipped by this system?