Tubes, magical wonderfull tubes. Tubes color and add character, they respond differently to dynamics and transients than solid state equipment, simply you could say that they saturate, but that's simpler than it really is. Some analogies here. Tube guitar amps, so you've got this wonderfull sounding tube amp, it would be unthinkable that a solid state amp, through a tube mic, or tube pre would sound anything like the "real" amp. The same logic follows for mics. Tube mics have a character that can't simply be added in later with tube processing, just like you can't make a non tube amp sound like a tube amp. I don't know enough to get into any technical reasons. When judging a mic, it's not really inportant weather it's tube or not, just if it sounds good.
-jhe