Your question seems to ask why there are different speed grades of memory available. As in, why wouldn't there just be one speed -> the fastest. Also, perhaps related is "why do the faster speed grades cost more, because I can overclock the slower stuff and it really is the same chip, right!?"
One of the other answers painted the reasoning behind this as strictly "marketing". This is part of it, perhaps, but there are solid technical / physics reasons for this as well.
Here the deal: When semiconductor devices are made, there is actually a tremendous amount of variability in the whole process. That is, even though the whole process is the same for each wafer run of devices, each individual part comes out somewhat different. Not only do some work and some not work, but also some will ultimately work to different levels of performance based on voltage, temperature, power usage, clock speed, etc.
After a few wafer runs of a given type of part are made, the semiconductor vendor will have a notion of what their yield curve looks like to various sets of test conditions. They then use a statistical analysis to define a set of performance bins that each individual part complies with.....in effect the slow and the faster speed bins. For parts made in large volumes, there are usually several different possible bins and many possible combinations of test conditions the chips are labeled to comply with.
So for memory parts, a given device may comply under all test conditions at 600Mhz, but not at 700Mhz, so the part goes into the 600Mhz bin. A part that complies with everything at 700Mhz, but not at 800Mhz, goes in the 700Mhz bin, etc.
This all conforms to a distribution curve and you can see for progressively higher speed bins, less and less parts are going to comply with the tighter specs of the higher speeds. In effect, the higher speed parts are more scarce, therefore they can command a higher price for the people that really want them. Conversely, you can see that they can sell slower parts a lower cost because they are in effect easier to make.
Summarizing: In the end, this comes down to the variability in the manufacturing process, statistics and some basic economics of supply and demand.