[ubernostrum]

why didn't NASA or anybody else do it before?

Why haven't we gone to the moon since the 1970s? Because there wasn't the political will to keep funding the programs that went there. Why was the Space Shuttle such an awkward and ultimately non-viable way to go to space? Because there wasn't the political will to fund genuine reusable launch technology. SpaceX's big advantages are in having the hard up-front R&D already done for it by someone else, and in not having to rely solely on political decisions for the existence of its market; even if there are no more nice-PR resupply missions to the ISS, there are still going to be private actors who want to launch satellites.