Because it is in essence an embarrassment for them.
They thought that nobody liked Snyder's films (which is a fair assessment if we go by BvS's theatrical run) but people did love the ultimate edition. Justice League turned out a flop. Wanna know what made both BvS and Justice League flops? Studio intervention. That and Zack's daughter's death during the latter film which led to Whedon taking over.
Some people really wanted to see Zack's vision in its entirety because the dude does have ambition. We don't see a lot of that in this genre anymore except in films like Logan, Joker, the Unbreakable, Raimi and Nolan trilogies.
Due to the consistent love that goes towards Zack's ultimate and directors cuts, they were willing to cave in to bringing Justice League back in (mostly) the original vision laid out by Zack years ago, prompted by the many fans on twitter.
However, it's been half a decade since the Joss cut of Justice League came out. They've got to play their cards right if they want to keep afloat in the cinematic world. I hate that studios only care about exploiting our favourite characters for profit, but that's the way it's been for a long time. They've already been in the process of moving on for a while which is we're getting the new Batman film etc.