@aidan,
Because I can remember Sundays before kids. I'd get up early, take the dog for a walk, get a newspaper, come home, read the paper for a while, take a nap, get up - go out for brunch, maybe take a bike ride, come home, read more of the paper, take a nap, get up, maybe go to happy hour and a movie, come home, do laundry, maybe vacuum, go to bed and read.
I LOVED it...I'm looking forward to being in that space again someday. Maybe your wife is just overwhelmed with everything. It's really hard to give up everything you want to do to do what you have to do.
But if she's a mom, she should - plain and simple.
Maybe you could get your child to ask her to turn off the tv and go to the park or something.