As was mentioned, a lot of archival tapings simply aren't broadcast quality, particularly for regional theaters. They can't afford rehearsed, sophisticated multi-camera shoots that may span more than one performances as would be typical for broadcast tapings. These are often one camera, back of the house, one performance takes. They were never meant to be seen by the general public. They're archives, used for reference and sometimes cut up for limited promotional use.
Also: this isn't just an Equity thing. Once things are intended to be broadcast or distributed, SAG-AFTRA gets involved. Any sort of recording that goes beyond archival or very limited promotional purposes usually requires appropriate SAG-AFTRA rates. Once things move beyond live performance, it's a jurisdictional issue.
(To say nothing of any other union or agreements that would be involved here, such as those representing directors, writers, designers, etc).
A lot of people are involved in making a theatre production, and I would guess most of those people did not sign on to have their work taped for distribution. And if the theatre is making revenue off it, it's not unreasonable for those people to want their due.
"...everyone finally shut up, and the audience could enjoy the beginning of the Anatevka Pogram in peace."