Not sure where you got that idea from. In the early 1970's they had nothing but open land west of Magic Kingdom to expand. Much of it was slated for Western River Expedition, but the area to the west of Adventureland was also wide open when they began Pirates construction in 1972.
Satellite imagery doesn't exist from 1972, but here we see an aerial shot of the western half of Magic Kingdom Park circa 1972. The land that Big Thunder is on has been cleared for Western River Expedition, and the area south of it where Pirates will go is wide open. There is no "backstage" buildings there at all. Nothing but marsh and trees zoned for park expansion.
Fast forward to the 21st century, and we have satellite imagery and a gaggle of parade and support buildings built in the 1980's and 1990's. Even when you take those out of the mix, there was and still is a huge swath of developable land southwest of Adventureland where they could have built Pirates on. They could have done a Pirates of the Caribbean that was TRIPLE the size of the Disneyland original, and still had a wide buffer of trees to hide it from Seven Seas Lagoon.
View attachment 326370
But they chose not to use that land. There's are reasons why, well documented in books and blogs and Imagineering interviews. But the choice was purposeful in '72, and WDW got a truncated 8 minute version of the 15 minute long Disneyland original.
But saying the Imagineers and Orlando execs were "blocked by backstage buildings" that didn't exist in 1972 is not the reason why.