You're right. After running through several dramas within the Learn Mode collection, it seems that VIki's default is the Simplified format, even for dramas with origins like Taiwan and Hong Kong, which currently still use the Traditional format. I think unless each specific Chinese drama, wherever it originates from, has its own Other Language Moderator for Chinese Traditional language, the default, if enabled by Viki, would be in Simplified format.
You can write in to Viki to make your request. I believe it wouldn't be impossible for Viki to enable an option to choose between Simplified or Traditional language under Learn Mode.
Btw, don't ever give up on learning the Traditional format, even if it may seem tougher than the Simplified, or you'll lose out on learning the more profound roots of the language and its culture (which I personally find very interesting and insightful).