I think it may be because he refuses to eat any “mouse” food from the pet store - he doesn't like cheese - etc.
I feed him pretty much what he might find on his own - and his favorites are greens - especially dandelions, clover, spinach and bronze lettuce - and radish tops.
His favorite of all is salted pumpkin seeds...and he likes walnuts and cashews and raw peanuts in the shell.
And he doesn't like American cereals - but I brought back some from Canada, which he likes. (Canada's foods are much healthier)
He loves teasing the cat - who will still try to get through the glass - while he sits there and laughs.
He's the longest living mouse I've ever known - and with five kids and 15 grandkids - there's been more than a few...