I've played golf since the '60s, and worked in maitenance since the '70s, and I've seen a gradual change over the years. It would need a book to tell of all the details, but there were things like improved equipment, hydraulic mowers to take the place of ground and belt driven ones, improved verticutting and aerating units, more sophsitacted sprayers, etc. The introduction of the string trimmer led to a paradigm shift in manicuring.
A large part has been played by new varieties of turfgrass, especially for greens like the bentgrasses and "superdwarf" bermudagrasses.
Chemistry has progressed by leaps and bounds, there are many times the products available today than there was in the '70s, especially in the U.S. Since the '90s growth regulators, unknown for turf in the '70s, have played a huge role in course management.
The general plant and soil science is always progressing, too.
But mostly, I think improved conditions were driven by market competition, each club trying to one-up the other, spending more money on everything, including labor, and simply paying more attention to detail. For example, 40 years ago roughs almost didn't count, and nowadays they are usually watered, fertilized, and sprayed like everything else. Bunker edges have gone from being ignored to becoming a focal point, and, like I said, string trimmers and flymos have done wonders for manicuring.