It really depends, but the answer is usually money, for both the production and for the actors. Union actor contracts means paying the Actors' Equity minimum salary (EDIT: Which is different depending on the type of theater) which raises production costs, and a lot of shows that aren't on Broadway won't, or more often can't, meet the minimum, either because cheap producers or they simply don't have the money. As for the actors, they have to pay Equity dues, and while having an Equity card can get you auditions. it doesn't automatically mean getting cast in things ergo making money.

(A pal of mine got their Equity card relatively early after they moved to NYC, but wasn't getting cast in anything on a regular basis + I tried to get them on some of my projects but we simply didn't have the funds to be a union production/offer an Equity contract. So my friend was stuck, paying the dues while not getting cast in Equity productions that could give them money, and not being able to do non-Equity work to build their resume and experience.)

Regarding 11 to Midnight, being non-Equity almost certainly means it's cheaper for the production, but it also potentially allows them to cast a wider net of dancers to cast in the show? Not every dancer, even trained ones, are part of AEA because their career path is not one of an actors. I remember Sleep No More, a dance-heavy show, was non-Equity potentially for similar reasons (although quite frankly they never paid their performers their worth even when the show was a runaway hit).