If you use a normal B&W developer at normal times and temps you will get black slides due to developing a good neg, but what's left is not just the pos in silver halide, but pos+whole lot more silver halide. It took Xtol at 1+1, for 40 degrees celsius at 40 minutes to produce good dMin (essentially develop out all the silver halide in the highlights so that there is nothing much left that could create density in the highlights at the colour dev stage) with E-6

E-6 FD has high solvent activity from the thiocyanate, one can also use thiosulphate in a developer (though more of it than thiocyanate) to clear the highlights at more normal processing times etc.