The Chinese government cannot dictate what Hong Kong companies do in Hong Kong. It is outside their jurisdiction.
The 3G networks in HK are not 'trial' networks. They were launched back in 2003 & the licences were auctioned back in 2000. Mobile operators in HK will not choose TD-SCDMA over WCDMA as the feature set is a downgrade for them. Consumers won't be interested either due to unattractive handsets.