People understand the platform better, no need to lure famous people to the site, there are other ways to verify (e.g., link to twitter account on web page), many more people across the spectrum of "famous" so it's hard to button down who/what is famous enough for the checkmark, global audience means famous in one place != famous in another, no consistency in how verified actually works, verified accounts became Star-Bellied Sneetches, etc.