I suspect that two factors here may be that Tapas has fewer users overall than Webtoon, and possibly that the way titles are displayed on Tapas maybe doesn't use an H1 html tag, making it overall weaker for SEO than Webtoon.
Another factor could be a lack of traffic linking to your comic. Do you regularly link to it on social media? Do you have a website that links to it? Have you linked to it on any interviews elsewhere, Kickstarters etc.? That said, this does seem to have limited effectiveness...
In a thread a month or so ago, I searched my name and comic title and it was visible, though lower down even than my webtoon (barely even a second mirror and I almost never link to it).
Okay, let's try "Errant Comic".... eeggghhhhhh.... 
The only surefire way to get the Tapas link for me is:
But that's with 3+ years of updates, a website, kickstarter and stuff all linking to it regularly and over 2500 subs. I think maybe better SEO is in order...
Perhaps due to focusing more on the App, Tapas haven't really considered the SEO of their website? It's a little frustrating when I'm working hard on my end to try to bring new people to the site in order to boost my readership, but I can barely even search the site because I just get local tapas bars (and don't get me wrong, I love a bit of delicious tapas, but...).