I think a big part is the looks. The SM58 is not visually distinct from a $10 mic for a karaoke machine. The integrated pop filter means you don't even get to have the "super professional" look of a pop filter a few inches in front of your LDC.
I considered making a box that accepts inputs from dynamic mics, adds some warmth, and has a USB output, to make it easier to get the LDC sound from something like an SM58, but I think nobody would buy it.