I'm going to guess it is to keep it slim. And while I normally think this fad to make everything thin is stupid, in this one case I feel it makes sense. You want the thing you're writing on to protrude from the table as little as possible.
I have the fact that especially "pro" tablets are becoming heavier. They're not comfortable to hold.
In my opinion the "perfect" tablet should not weigh more than 250-300 grams, we just don't have the tech for a flagship tablet that can weigh that much, yet.
Experimentally, I've found that my Kindle (320 grams including case) hurts a lot less than my iPad (470 grams) which hurts a lot less than my Surface Pro 4 (790 grams) when dropped on my face.