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As referenced in this question, there were a number of early spaceflight missions at higher inclination than may be expected (given limited payload capability, higher inclination further limits payload to orbit).

Skylab was launched from KSC (approx. 28.5 degrees N latitude); the maximum payload to orbit should be achieved with an inclination almost exactly equal to the latitude. So how/why did "they" choose to put Skylab into a much higher inclination orbit?

Skylab's science experiments included Earth surface observations, and the higher inclination orbit allowed more surface to be viewed. Per Living and Working In Space: The NASA History of Skylab:

The requirements of the earth-resource experiments caused major changes to mission plans. Primary among these was an increase in orbital inclination to 50º. Skylab would now go as far north as Vancouver, Winnipeg, Bastogne, Frankfurt am Main, Kharkov, Mongolia, and Sakhalin Island north of Japan. To the south, Skylab would pass over all of Australia and Africa and most of South America, except Tierra del Fuego. Three fourths of the earth's surface would lie under Skylab's path, the area where 90% of its population lives and 80% of its food is produced.

This diagram from the book dramatically illustrates the coverage at 50º inclination. For comparison, I've added a blue shaded area illustrating the coverage obtained by an approximately 28º inclination max-payload-from-Cape orbit; it's particularly striking that almost none of the US is covered at that inclination.