The biggest demographic on Tapastic are 13-30 females from America with Gamercat pulling in the most traffic to the site. This could also be why cartoony series get more interest since Gamercat is the biggest series on the site and it's a cartoony gag-a-day.
However, there are some very popular more tightly drawn comics too like @zelkats Undying Happiness.
Gag-a-days by their very nature are easy to spread and go viral, and cartoony story driven series tend to be more expressive, experimental, and fun and realistic series can be very serious, dramatic, and tend to take more commitment to read and keep up with.
So there are some factors beyond just the site's demographic that plays a part in cartoony stuff getting more attention.
However, Tapastic has a great mix of everything. Like Liar's Dice does an amazing job of mixing both realistic beautiful art with playful witty humor.
Also the biggest challenge with posting on Tapastic is that the artists themselves HAVE to self-promote inorder to be seen. Just posting to the site with no promoting at all will get that series no where (unless there's an existing fanbase that follows the artist here).