There are several reasons as to why RIOT would invest so much into China as to give them their own region and 4 Champs slots.
Playerbase: Large population = large playerbase = lots of money = more company funding
Infrastructure: China has the technology to support VCT level events at the highest level (best production)
Culture: Esports culture is very popular and being an Esports player is a very viable job. This compliments the fact that there are large companies that can gain sponsors and reputation/public interest from creating esport teams and competing in various games.
I'm sure there are more reasons but these are the main ones I could think of.