There are downsides to making things uber-thin.
- Build quality
A consistent complaint I've heard from iPad2 users is that, without a cover to add some thickness, the device can be uncomfortable to hold for any extended period of time. I feel the same about my iPad. The thinner the device the less room for flex the device has when dropped and the more likely something important will break. This is also important when considering build material. Aluminum is pretty, but it is easy to permanently damage the material. Plastic, as lackluster as it is to touch, is very resilient.
The smaller you attempt to make something the more likely you are to have to use newer smaller parts, new manufacturing techniques, and new build processes. Smaller parts also means less internal space, less gaps between components, and higher precision required. This immediately increases cost. Defective items have to be refurbished creating extra work. It also affects build quality in that you are using techniques and processes that might not have been fully baked yet or parts that may not have had time to shake the bugs out. There's risk involved.
I feel it highly unlikely that Samsung saw the iPad2 come out and immediately went into redesign mode and created a smaller device. It's more likely that this device has been in the works for a year or more, given the time it takes to align designs, get FCC/legal clearance, order supplies and align partners.