Top-loading dishwashers were most likely phased out for the classic corporate reason - demand fell and it was no longer cost-effective to make them.
Just going out on a limb here, but I would guess that most folks found a front-loader was easier to deal with and that many kitchens from the late 50s onward had or had the space for a standard 24-inch wide front loader. I would also guess that as multi-level wash systems evolved, a top-load design was awkward with a full-size upper rack wash arm (I had an Inglis/Whirlpool top loader that had such a system and I found it harder to load than my one-arm-wonder Whirlpool!)
The top-loader was good solution for a narrow kitchen, though and I personally like the top load style because any liquids or stuff left in cups or on plates will fall down into the tub bottom.
Several manufacturers did indeed offer both front-load and top-load dishwashers - but most top-loaders were portable units. GE, Whirlpool, KitchenAid, Maytag and D&M (who made Kenmore and other brands until the 80s) were examples. GE phased out their built-in version of a top-load dishwasher by 1963, however Westinghouse offered a built-in top-load unit for longer (not sure what year they dropped the design). Top-loaders seemed to disappear altogether by the early 70s. Oddly enough, KitchenAid offered a built-in top-loading dishwasher called the Brivia in the early 00's - it looks as if it was marketed as a second, utility dishwasher for preparation items or auxillary kitchens. And yes, I admit I planned for one in my kitchen in Ogden, but by the time I got around to doing those renovations the machine had been discontinued (they were not very good performers apparently).
The machine in today's POD was a classic overenGinEering example from the folks in Appliance Park - a 30-inch wide dishwasher may have seemed like a good idea at the time (more space for dishes, bowls, pots and pans) but I can't imagine a single bow-tie impeller getting enough water into the corner of the upper rack. I believe GE compensated by using more water. Flaws aside, if I ever had the chance to get one of these GE's, I'd get it!!