Part of the reason was the push in the 90's by Robert Reich, Clinton's Secretary of Labor, who, in response to globalization and outsourcing, was essentially arguing that we need to turn the whole US workforce into a managerial class in. This required putting a far greater percentage of high school graduates into academic college programs. Technical jobs and manual labor were de-emphasized.
Another possible cause (although I risk the fate of Larry Sumner by suggesting it) is that the higher echelons of community college administration (at least in Colorado) have become dominated by women. They don't identify as well with plumbers, or mechanics. Engines are greasy and smelly. It was a female president who closed down our diesel, auto, and hydraulic pneumatic programs. We do however, still have one of the finest Woodworking programs in the country. Maybe it's because look good in a living room or a college foyer.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13