Rlawry is correct in explaining that additional gain leads to a decrease in S/N ratio. Typically, the total gain needed by a MM cartridge is around 50 dB, whereas an MC cartridge needs 70 dB or more. Due to complex laws of physics, the amount of engineering effort necessary to maintain S/N ratios with this 20 dB extra gain is enormous.
On the other hand, the EIA published a standard for measuring S/R ratios (among other things), as a common ground to compare the noise performance of preamplifiers. In the case of Phono MC preamps, the standard demands the S/N ratio to be measured with an input signal of 0.5 mVrms at 1 kHz, and an input termination of 10 ohms. Most manufacturers simply ignore this standard, prefering their own way of expressing the S/N ratio trying to favor their product. The problem is, a S/N ratio has absolutely no meaning unless the input level and termination are explicitely defined. For example, a preamp could be described as having 80 db of signal/noise ratio with a 5 mV input signal. The same unit would have a 60 dB S/N ratio with a more realistic 0.5 mV input. A very audible 20 dB difference, no less! This could explain the discrepancies mentioned by Aquaanox.
Some people try to minimize the importance of having very high S/R ratios in audio playback, but I think this is wrong. After all, noise is not part of the music and minimizing it can have a vast improvement in detail and ambience retrieval, leading to more realism in musical playback.