Because the 'blad 500 (and its Japanese cousin the Bronica SQ) was pretty much the workhorse camera of pretty much anyone in the business from the 70's through to digital becoming worthwhile to use back then, capable of shooting weddings on a weekend, and portrait stuff in the studio through the week. Back then product cycles were in decades, not months, so if you bought a 'blad in the 70's, in the late 80's you'd still be holding onto it waiting for those new digital things to get beyond a couple of mega-pixels so you could go with them - same reason that 35mm wasn't in that wide a usage - on film, you weren't likely to be printing much larger than ideally 10x8 from 35mm. Plus, cropping was pretty much a given anyway, as it was largely wet-prints from an enlarger setup.
Digital scanning WAS a thing, but it was largely the province of print magazines and newspapers, and was be ferociously expensive, so generally outside the scope of non commercial projects. That said, the quality from a 10x8 transparency, drum scanned was absolutely INCREDIBLE - and was quite a kick when you saw something you'd done in a proper glossy magazine... Especially if you'd shot two frames and retained one of them so you'd got your own miniature stained glass window...