Actually there is a reason for Tapatalk that has nothing to do with design.
Some people use Tapatalk as one place (app) to stay up to date on multiple forums.
It's like having an RSS feed vs. having to visit dozens of news sites a day just to check if anything new was posted.
Oh, and Tapatalk is free for forum owners. The $69 quoted above is for a branded design (and it says exactly so on that page).
I'm not employed by or affiliated with Taptalk, but a forum owner.
Currently running SMF. Would love to have a more modern design and social logins, both of which your software has, but many of my users rely on Tapatalk.
PS - love your much more reasonable browser requirements (IE8+ vs. IE10+ for Discourse).