Because he couldn't even if he had wanted it.The only thing that was obvious to me about Wawrinka was that the BO5 format suited him better than BO3, as he often needed a lot of match play (within a single match and throughout the tournament) to be able to work himself into his best form, and BO3 did not allow that. But the whole "he doesn't care about masters" thing is a huge cope by his fans to justify his relatively poor performance outside of slams. I don't buy it for a second. Even the Big 3, with all the titles they've racked up, still work hard and clearly care to win masters titles most of the time. Why wouldn't he? Especially when he only has one Masters title lol
It takes a lot of mental & physical energy and stamina to win both masters and slams, especially those close to each other like Rome & RG (+ Wimb around the corner), or Can/Cinci & USO, and there is a high probability for picking up some niggling or serious injury after inevitable consecutive QF/SF/F masters matches vs Big4.
Young Murray used to win many masters but was losing in slam the QF/SF/F.
Under Lendl he completely shunned masters and started winning slams, and Murray was a physical beast compared to Stan and also younger.
Sampras was certainly capable of winning HC masters but in his last five seasons he won two masters and four slams.
In his last three seasons he entered three USO finals (F,F,W) but just one QF in Canada and one 3R in Cinci as the best results therein.