I can't say for sure but...
Everything in aviation costs about 20 times as much as it cost on the ground (no kidding).
Anything which broadcasts or receives a signal could potentially play havoc with aircraft systems (hence no phones / radios etc. allowed).
Have you seen how much fuss has been made about using mobile phones on a plane? EVERYONE wants to use their phone and they'd (grudgingly) pay high call costs, so there's plenty of money to be made... if only they could get a system certified.
A hearing loop system would also cost an arm and a leg to develop, certify and install. But unlike the phone system, how many passengers would actually use it? And how many of them would pay for it?
It's much cheaper and just as effective (from a safety point of view) for the airline to give you a breifing card and then a stewardess can go through it with you personally.
HTH