The incentives just aren't there. It's a lose-lose-lose for Universal.
Universal wouldn't have agreed to feature the IP in the first place if they didn't think it would be a draw of some kind (or it services a relationship with a company they want to do business with in the future. Take Evil Dead now, get something like It down the line.)
Your assumption that they didn't put too much money into designing anything Evil Dead-specific is a false one. Everything IP-related has to get final approval from the party/parties that control the IP, and Universal's whole pitch is to realize the film worlds in a physical space. In other words, they absolutely put the usual amount of resources and effort into building/sourcing sets, props, costumes, makeups, and masks that closely resembled what's on screen. Recycling is more permissible/feasible for original houses, though obviously props are props are props, so some things can get repurposed in IP settings.
The merchandise would be another issue, but probably not the biggest one. With that said, reprinting house shirts to exclude Evil Dead Burn this late in the game would be a considerable expense and hassle.
The actual problem would be replacing the house itself. The Horror Nights design and production workflow is not built in a way to facilitate last-minute pivots, period. These houses call for something like 15+ months of planning from conception to execution (not sure what the actual number is these days, but for a while, it was around here). Yes, they've already built sets, which now need to be modified enough to conceal what they originally were to the satisfaction of WB. It's hard to say how much of a lift that is without having seen the movie or what they've constructed, but the event starts in just over a month.
This means they would have to rally creative teams of all stripes to quickly brainstorm a new concept that can work with what's there for the least amount of money, because they didn't budget for this. These teams are also swamped with actually finishing the rest of the event, and some vendors may have already committed to other projects. Still, they somehow either need to build (or more likely, again, recycle) costume, mask, and makeup assets that ideally align with the casts they've already assembled. Then they've gotta clear the new house name and description and commission new art assets...
...all of this for a haunted house that they know is more likely than not to be a dud and rejected by guests. Hellblock Horror was not good, and word got around fast.