In my JAR Perf-A groundschool years ago, the instructor, an ex DC-8, DC-10 loadmaster, flight engineer, general dogsbody who'd spent the vast majority of his 10,000 hours flying around Africa claimed it was because 35' was all the original 707 could achieve off a wet runway, engine out, and with the FAA (or CAB I guess) back then being in the pocket of Big Boeing, they bent the rules to suit.
Good story, if nothing else.