Maybe because white recall the snow and the warm soft fur of some winter outfits XD It could be useful wear a warm shiro loli dress in cold months like December or January, I think it would be a pain go out with a light dress with bare arms/legs while it's 5° XD
How come whenever I see shiro-lolita, it's almost always something really winter-y with a lot of fur? White isn't a winter color.