What mechanism causes a butter crock to function better than other options?
Butter, like most fats, is actually quite resilient to microbes. The problem you are more likely to experience is rancidity. Fat goes rancid by oxidation. Exposure to light, heat, and air cause oxidation and accelerate the process of butter going rancid. 

Putting butter in your refrigerator addresses the heat and light, but does nothing for the air. The paper that butter sticks are wrapped in are quite porous. You can see evidence of this as a rind that develops on your butter after a couple of weeks. Butter can also absorb odors in your refrigerator through this wrapper.

A butter crock, as shown in the accepted answer to this question actually serves to protect the butter from air, light, and heat. Butter does not need to be kept below 40 F to stay fresh. The butter crock referenced there will keep butter fresh up to temperatures of 80 F, and likely higher, but the butter will be too soft and slip out at higher temperatures.

A plastic or ceramic butter dish are similar in function to the paper that wraps butter. Ceramic is unlikely to seal at all, so the butter will still be exposed to air. Plastic, while capable of a great seal, will still trap oxygen in the container with the butter, allowing oxidation to occur.

A butter crock really does seem to be the best of all worlds. You get easy access to soft butter, with little risk. Mold will eventually grow on the butter, but this should take weeks at room temperature. If you don't use a stick of butter within two weeks, you're doing it wrong. :-D