There are many reasons to why oolong tea is so much better, but not popular.
1. Small production, only 3 regions in 2 provinces produce oolong tea in China. Taiwanese oolong teas are not widely available in China. Majority of the country of China produce green tea.
2. Complicated process also limit oolong production.
3. Transportation in the old days took much longer, shelf life was a big concern, light fermented teas could not make it else where and maintain its freshness simultaneously. In recent days, Tie Guan Yin gained popularity after a face lift and benefiting from refrigerated shipping and storage.
4. Local teacoholics consume most of the oolong teas before the tea had a chance to get out of town. City of Guang Zhou population consume an average of 2.5 kg annually, Chao Zhou population consume an average of 4.5 kg of tea annually, the entire China consume only 1/4 of a kilo of tea annually.
5. Complicated brewing method. Kung fu tea is a tradition in the southern states, west to Yunnan, east to Fujian, but not for the rest of the country. Si Chuan mainly used gaiwan as drinking vessel, later adopted by the rest of the country.