I think there are a few reasons why C-dramas often don't get subbed.
1. China's large population. They can make enough money just from Chinese people, so there's less motivation to market C-dramas internationally. Japan and Korea have smaller populations, so they need an international audience.
2. The types of C-dramas that get subbed are too narrow, so they appeal to a more narrow audience. If you browse the C-dramas on Viki, for example, you'll get the impression that the majority of C-dramas are historical dramas starring young good-looking actors. People who don't like that type of drama will probably think "C-dramas aren't for me."
There are actually many different types of C-dramas, but most of them don't get subbed. They don't get subbed because they're not popular internationally, and they're not popular internationally because they don't get subbed. Some C-dramas do get subbed but don't get promoted, so K-drama fans don't even know that they exist.
3. C-dramas tend to be longer than J-dramas and K-dramas, so people who are new to C-dramas may be afraid to make the commitment.
I think C-dramas will eventually become as popular as K-dramas, but it might take 5-10 more years. I think streaming websites need to license a good selection of high-quality C-dramas from a variety of genres (e.g. crime, thriller, modern fantasy), and more dramas with older main characters, and do a good job of promoting them.
It might also help if more people start blogging about C-dramas.
