I have to agree it has to do with the newness. Almost no one in my friends circle knew about Tapastic, but they knew about LINE Webtoon. When I posted a comic with one page marked NSFW, a lot of my friends complained to me because they couldn't read it because they didn't have an account. XD I managed to convince some of them to get one.
I don't think Tapastic will end up like tumblr has, because whereas on tumblr you have lots of curators and reposters, Tapastic is focused upon original content, and people on here love original content. We also don't have an anonymous messaging system, and I'm thankful for that.
And also the design of the site! It's easy to use on desktop and the apps are super nice. And I love the font so much, I'm really tempted to use it for everything.