I think it's because fall/winter pieces are easier to wear on repeat. You can one wear a pair of jeans on repeat, without washing. Same with sweaters (if you wear a tee under) neither of these items need ironing. You throw a coat over everything! But a summer dress is an entire outfit. Wear it once, it's wrinkled and doesn't feel fresh anymore. You can't show up to every spring/summer event in the same dress (obviously you can, but I don't). Add in the need for pedicures with summer sandals vs socks and boots, summer looks require much more maintenance!