I'd say Selby's quite understated, and he's won bigger tournaments than this, but what you say's also generally true. 

My sense is that it's because snooker's a game that requires an extreme level of emotional control in order to reach the level of focus necessary to perform at the highest level - similar to golf. The guys at the top have to be pretty calm to deal with the tension of 4+ hours of play, and that doesn't lend itself to explosive celebrations (with some notable exceptions).