it's because Aldi don't stock them all year round, that they can do the prices that they offer.
They are able top bulk purchase on big shipment, get it sent in store in one go. And don't have the hassle of trying to re order when a size goes out of stock, trying to offer various options of colours and technical 'ability'.
They don't purchase much in the either very small or extra large sizes.
They don't have to employ staff to help give customers advice on fitting, breath ability & water 'proof-ness'.
bit like a running/bike/triathlon/camping/skiing shop does.