Isn't it obvious? It's because Wallace is simply a much better actor and 10000X better looking than Yan Kuan (who's looking kinda aged nowadays)? Sorry, i didn't find Yan Kuan appealing in the trailer ...but I'm open-minded. It's a pity Wallace couldn't be No. 1 male lead due to SARFT rules that a Mainlander has to be the male lead if the female lead is a foreigner.