Because Jon would object to anything Littlefinger-related. Sansa has become much more pragmatic than Jon, who would struggle to accept anything from somebody who, say, had a part in having his father executed.

Good to see Cersei's world turned upside down. She was getting too comfortable — shaking up her situation was the right thing to do. Karma aside, I'm eager to see how she reacts to her new circumstances. This is good soil for further character development, and I kinda hope she gets to fulfill her threat. I guess I…

Alternatively, she could prepare him a nice meal and then shoot him with a shotgun while he's eating.