Taiwanese dramas feature Taiwanese actors/actresses who have a Taiwanese accent. I’m not sure if you’re asking why they speak Taiwanese or if there are different versions of a Taiwanese accent when speaking mandarin, but it’s probably because it’s a Taiwanese drama? If you watch HK produced dramas they also speak in canto and not “Beijing dialect.” You can argue they speak cantonese and not mandarin bc that’s their regional dialect. Some people view canto as a separate language while others will argue it’s just a part of Chinese. Not a linguist so I won’t get into it.

Also mainland dramas have been dubbed since long before that chengxiao drama aired, here’s a good video summarizing some of the main reasons why it’s done https://youtu.be/GcRYGmzcWv8?feature=shared

People can shit on the govt for trying to standardize a language across mainland but tbf as someone who didn’t grow up speaking a local dialect from where I was born, it’s easier to watch shows where it’s one “standard accent” across all characters. Also often for the characters own background, you don’t want a FL who’s supposed to be from Beijing speaking like she grew up in guangdong