Why are there so many different types of lubricating grease? Why do some cost more than others?

There are many different types of lubricating grease because there are many different types of applications where grease is used.  Many greases are formulated to have specific properties unique to the application for which they are intended.  Electric motors take one general type of grease, while flexible couplings require a very different type, and automotive wheel bearings require yet another type of grease.  For more exotic applications, such as in aerospace, Arctic conditions, or high temperature bearings in the steel industry, there are specially formulated products that provide specific levels of performance.  Those products are often formulated with synthetic base fluids and contain relatively high concentrations of additives, making them somewhat more expensive than general purpose greases.  On a typical passenger car there may many different greases used.