ruprecht, certain understudies have been going on consistently at this point because they're not all rehearsed.
Jerry is rehearsing all the covers in "sessions," with them going one group at a time. The ones that have been on so far are those who have been getting time in the rehearsel room.
Annaleigh Ashford in particular has said to some of the fans that this is why she hasn't even been rehearsed yet for Elle, let alone having any prospect of going on anytime soon. Becky Gulsvig, in that case, has been rehearsed with the first batch of covers.
They're just starting to now give the others time too, but with the stress of opening the show and all the hooplah that came along with it, everyone involved figured it would be easier and alot less stressful to rehearse the understudies one group at a time rather than trying to teach multiple people principle roles at the same time.
I suggest you learn the facts before you give an acerbic reply.
Tonya Pinkins: Then we had a "Lot's Wife" last June that was my personal favorite. I'm still trying to get them to let me sing it at some performance where we get to sing an excerpt that's gone.
Tony Kushner: You can sing it at my funeral.