Yuri, correct me If I'm wrong, stopped trying around 2010/2011, didn't she? If yes, It may be because she became more known for her appeareance and started to constantly yo-yo diet. She became known for her honey thighs and having the best body in kpop during Genie era and then during Hoot she started to get really skinny. The general public's obsession with her apperearence probably made her think she only needed to work on that.
This is only my theory though.