Because the 21 carbon spoke hubs are heavy as shit
Really? Haha, 325g/pair on our 21/21H hubs, but still much lighter than DT Swiss hubs for carbon spokes, we don't use original DT Swiss hubs for carbon spokes, actually DT Swiss decided to make those hubs to supply to STREN who makes VONOA carbon spokes for many big brands, but firstly DT Swiss was too uncompromising on the price, secondly some brands don't want to use DT Swiss hubs for carbon spokes as well, so I believe you won't see current DT Swiss hubs for carbon spokes much longer.
Anyway, stable quality are most important, as I said when we released our 20/20H hubs for carbon spokes, we don't make them extremely light as some other companies do due to much higher spoke tension on carbon spokes. @Toxin, Have you seen anyone complaining about our 21/21H hubs except side entry so far? Maybe you are picky on weight of any part of bikes, but please don't say bad words on anything you haven't tried personally.