The scientific reason: carbonated drinks contain acids that interfere with our calcium absorbtion. It's counter-productive to your daily supplements.
The various other reasons: newly sleeved stomachs can have a great deal of trouble pushing gas out. Burping is hard to non existant in the beginning. You CAN belch later on, but it's not the same as it was before surgery. It takes more effort and isn't always easy. It can take a couple squeezes/heaves to work one up.
As far as stretching your stomach, no. It's a gas, and even if you have problems burping it up right away, it's not going to stay in your stomach long enough, nor will you physically be able to drink enough at once for it to cause you any stretching harm. What it CAN do, however, is act as a gateway to feeling hungry sooner (some people say diet soda causes them to graze) and can interfere with insulin reactions in some diabetics.
I do drink soda fairly regularly, but I always drink triple the amount of water per ounce as I had of soda, and at 96 pounds lost in 4.5 months, it's not interfering with my weight loss. My labs also came back just fine, as I am taking my supplements. Please don't use me as the norm, however. I know my body and mind and what they can handle. Your mileage will vary.