for your information, more than 65% of mainland series that are shown on mainland TV are modern series. It's just that people on this forum only care about their ancient series.
The reason that mainland modern series aren't that popular is because they are very culturalized. If you didn't grow up in mainland, you won't get a lot of the jokes mentioned in the modern series, and you won't understand some of the situations.
The same thing with some Taiwanese series. The only Taiwanese modern series that are popular in this forum are the idol series. However there are a whole other genre of Taiwanese modern series that focuses on everyday life in Taiwan and specifically Taiwanese culture. However, most of people outside of Taiwan haven't seen those series.