It's not off limits in the regulatory sense. Balloons are generally exempted due to their size and power constraints. Per the submission, the balloon weighs 2.4 lbs at launch. That doesn't give them much room to add a transponder and battery for ADS-B while staying within the target weight limit.

47 CFR 87.107 requires that radios transmitters in aircraft broadcast an identification number, and the allowed forms of identification aren't permissive enough for non-registered aircraft.

ADS-B out is still relatively new (especially in aviation terms) so I expect we'll see this continue to evolve.

It's not off-limits. But, it's not required (for most balloons, ULs, etc). And due to cost and/or weight, people don't always use it when it's not mandatory.