Consider that the component + socket stands about 0.8 cm off the component side of the board, and protrudes 2 cm below the wiring side. the spacing between slots in an IBM PC backplane is ~0.75" which is about 1.9 cm. That would not allow you to put a PCB next to a wire wrap card and would cause problems in a case such as the venerable APPLE-][ which had different addresses for each slot.

I found it very odd that 3-level sockets were used at all, since 3 levels were never needed. Nevertheless, most wire-wrap cards had 3-level pin lengths. I found this very annoying, for reasons already stated and because the longer pins tended to bend more easily.

RE