One reason is that the bass drivers used in most speakers today are so small (6-8", which hardly qualifies as a woofer in my opinion) that they don't have enough bass extension to allow such steep slopes. Ideally, the response of a speaker should extend an octave above or below the point where it is to be crossed over. If this is not the case, the rolloff of the speaker itself is added to the slope of the crossover netowrk. A crossover that has a 12dB/octave slope when used with a typical speaker, yields a much steeper slope than the crossover alone.