The -100 series designator has been unfashionable for many years now (it was missing from the 777 too).
The -100 designation was a place holder for a 'shrink' version of the 757, 767, and 777. However it's very difficult to shrink an aircraft and keep it economical (as Airbus discovered with the A318). You end up with something that's too heavy with too much wing that costs nearly as much to build as the original, but is much less valuable to a potential customer. Hence the -100 versions were never built.