Few reasons.. My Sitka gear was about 3 years old, so don't know if they changed anything, but the seams on the Jetstream jacket let wind and water through, and while I know the softshell isn't designed for rain, most the time it would still have water beading up on it, and on my shirt you could see where every seam was because it lets the water right through. On the Guide DCS jacket, the seams don't seem to let the wind or water in. I haven't had it in a full on all day down pour, but had it in snow and light rain and stayed dry. Feature wise I would call it a toss up (pit zips and pocket wise). On the pants, the Timberline is a fantastic pant, love the waterproof knees and seat as well as knee pads, one thing that drew me to the Attack was the zippers on the sides. Our archery season is down right hot, and on occasion gets up in to 90's, Timberline I get way hot in, and in the Attack I can open up the vents, and while I'm warm, my legs don't feel like they are baking like in the attack pant. I know they make thinner pants, but generally have 1 pair of pants. In terms of cold weather performance, I would call it a wash, had both down into the high teens and 20's and with a good base layer underneath didn't prefer one pant over the other.
On my Sitka gear Core lightweight long sleeve T, the glue gave up and the pocked half came off which kind of peaved me a bit as well. Kuiu gear is generally lighter, and the rain gear IMO is more affordable (I don't spend a ton of time in the rain so was looking for lightweight packable gear for the occasional cloudburst) and Sitka doesn't offer anything in the cheaper ultralight category.
Pricing for Kuiu is cheaper.. Granted Im grabbing MSRP from Sitka, which you can find cheaper, but still more expensive than Kuiu (if you are patient you can get Kuiu when they have sales 15% off)... Pricing examples...
Kuiu Attack Pant $129.99 (18.5 oz)
Sitka Timberline Pant $229.00 (32.98 oz)
Kiui Guide DCS Jacket $189.99 (24 oz)
Sitka Jetstream Jacket $329 (25.43 oz)
Luckily for me Kuiu us also about a hour drive so I got to cruise down to the showroom, and check out the gear. Staff was super friendly and knowledgeable, was able to tell them about the type of hunting I do and they recommended gear for me which was nice. A person came in with one of their backpacks and said he was having issues setting it up with his rifle, staff got him all squared away so he could detach his rifle without taking his bag off. I haven't had the backorder issues with them, but I know that is one knock, as well as being mail order only unless you live close to them....