The reason for the shorter Pirates
When the Magic Kingdom opened, many guests had questioned where the Pirates of the Caribbean ride was. Disney had vastly underestimated the appeal the ride had gained thanks to being shown on The Wonderful World of Disney TV show. There was also how scores of WDW guests were not Florida residents and not willing to drive to the coasts for pirate-themed spots that lacked the flair of the Disney ride.
With such public demand, Disney execs decided they had to have Pirates put in fast. And by "fast," they were soon printing buttons declaring it was coming in less than two years.
Davis tried to fight this, arguing that as soon as Western River Expedition opened, guests would forget about Pirates. But the execs were also pushed by the energy crisis of the early 1970s harming the tourist industry and thus hitting their budgets.
Davis at least said they could try to update the ride a bit with ideas like an effect of a pirate turning into a skeleton in the moonlight. While that never came to be, it would be an obvious inspiration for the blockbuster films.
The differences for Pirates
The big challenge for Imagineers was finding a spot for Pirates. They settled on creating a "Caribbean Plaza" for Adventureland and crafted a very compelling fort for the queue area with bits like pirate skeletons playing chess.
For the ride itself, because Florida had a lower water table and there was already so much space around the area being used, Imagineers could only do one short waterfall drop, not two as in California. It also led to the ride being made a bit shorter, absent a couple of the opening cavern scenes. Davis did manage to eliminate the "rampway up" to the docking area and instead a separate unloading dock for guests.
Disney has managed to update the ride over the years, yet one can't help but wonder that if Disney had just planned to put Pirates in WDW in the first place, it would be an even better ride with the Western River Expedition by its side.