A lot of times, creators promote their works and publish at other places in addition to Webtoons. They might go to Tapas, Smackjeeves, Webcomics App, even social medias like Twitter, Tumblr, or Facebook.
Bottom line -- they've built a following elsewhere (which may have followed them to Webtoons), and they've promoted their abilities on other social medias. You don't necessarily have to be on Webtoons a good while to be featured there.
Some people chalk it up to luck, and that can play a part. But a lot of times, you're not seeing the Behind-the-Scenes -- which includes the questions:
"How did they promote themselves?"
"Did they post other works elsewhere?"
"How did they network with other people?"
"Is this their first comic or did they create other works before this?"