Because the DCEU is not popular. If the DCEU was going strong then Batman interacting with it would only benefit the box office.
Like imagine if we were coming off smash hit Superman and a really beloved Justice League, this exact same Batman movie, with maybe a couple of references to said films, would be doing even bigger numbers I think. The fact that it still is going to do very well is just because Batman is a very popular character with a high floor, and probably because it feels distinct from the Affleck incarnation (moreso than Holland's felt from Garfield's).