It likely has more to do with PR agencies than anything.

Being "successful" isn't necessarily the point, and I don't think it needs to be. If the goal were to write about the most successful companies, there would probably never be a startup mentioned--and it would probably be an article you had no interest in reading. "Google, still huge. Exxon, yup they make a ton of money. Etc etc."

Well is this part of that 30 under 30 thing they do?

There's nothing wrong with focusing on a particular demographic.