The Battle Bridge would have been used only during the saucer separation mode, but this feature was rarely seen on screen
Saucer separation was planned as a regular feature during the early days of Star Trek: The Next Generation. Several story lines, including a B-plot for "When The Bough Breaks", were to use saucer separation. However, budget limitations at the time did not allow for extensive footage of the separated components to be shot, and it was also felt that separation slowed the progress of the story.
From Memory Alpha
Anyway, the set of the Ent-D Battle Bridge was redressed in multiple occasions to be used as the Main Bridge of other starships. Since the use of the Battle Bridge would be useful only during the saucer separation mode, I think that the production preferred to use an otherwise unused set for other purposes, and was probably deemed that once the Battle Bridge set was dismissed, there was no need to reassemble it, but instead leave it free for other purposes.
By the end of the third season of The Next Generation, the battle bridge set had also been redressed as the bridge of the Miranda-class USS Lantree in "Unnatural Selection", the bridge of the Ambassador-class USS Enterprise-C in "Yesterday's Enterprise", Starbase 173's courtroom in "The Measure Of A Man", and the cybernetics lab aboard the Enterprise-D in "The Offspring".
From Memory Alpha