One good question is why it took so long for foils to show up on sailboats. This ferry was in use off Buenos Aires over 40 years ago.
It didn't take so long.
Dave Keiper's cruising tri Williwaw was fully flying in the ocean in the late '60s. The video of the boat in action are shocking - the hull and rig looks extremely crude, about as efficient as an early Piver, but the boat smoked when foiling fast.
Small cats like Icarus and Mayfly were foiling at Weymouth Speed Week in the '70s, and there were many other foilers at such events.
Tabarly's tri Paul Ricard was using foils, adjusted by a pivoting main beam, when it was launched in '79 and other tris such as VSD were fitted with them for some time. The 26m (85' IIRC) tri Charles Heidsick was designed to semi-fly using foils and the ground effect of the beam.
Moths were, IIRC, foiling in the '60s in trials; there's blurry pics on Moth sites of a scow foiling on (IIRC) Port Philip Bay in Australia in that era, with further details about the experiment. Windsurfers were photographed foiling about '77, for example in the Churchulla (sp) Bros book. There was also foiling Mistral M1 at my local beach around '83 and there are pics of Harken stock foils being used and advertised for sale on the original Windsurfer around the same time. And as Doug has pointed out, Monitor was there in the '50s.
With the technology of the day, though, boats were normally too heavy to make foiling effective around a course. Creating sails that were powerful enough to lift a boat onto foils but could then stay flat enough when the apparent wind increased would have been impossible, too.
You can add in the fact that popular craft tend to be simple, tough, cheap and perform well in light winds, which are not easy things to achieve on a foiler.
The fact that offshore boats were foiling decades ago proves that foiling trickled UP to the America's Cup, like most technologies did.