Speedplay acheive their remarkable weight claims and high ground clearance by building their pedals the opposite way around to most other manufacturers (Look and Shimano come to mind). Speedplay pedals have the main pedalling platform built into the cleat that attaches to the shoe. Other manufacturers prefer to build smaller, lighter and cheaper cleats, and put most of the structure into the pedal.
And because you sooner or later have to stand on any cleat (even if it's only at traffic lights or before a start gun), speedplay aren't hugely popular among those who can't afford to constantly replace their equipment.
That said, there is a fairly large demand for them among those who can - they have a phenomenal amount of float. This makes adjusting them perfectly less of an issue, and means they're much kinder to those with sensitive knees then competing brands.