Depending on the shoe/model the cost can actually be pretty high to add another extreme size and in a lot of cases, the ROI just isn't there. For every size (or at least size break) that you make, generally you need a mold for the outsole, a mold for the midsole, dies to cut the upper parts, plates/pin sets for stitching the uppers together. lasts for the uppers, and potentially some other random parts. This will typically run in the tens of thousands of dollars. Not to mention the increased scrap rates associated with shorter runs.
I'd say the ROI is usually worth it for sizes in the men's 6-12 range because you can usually use most of the same parts for men's and women's models justifying sizes 6-8. Outside of that you're typically losing money on a running shoe, especially flats or spikes.
It makes way more sense for shoe companies to limit the extreme sizes to fewer models and force that smaller population towards them. This gives them a higher demand on those models (instead of having low demand on several models) and limits the investment. So they at least have a chance of turning a profit on them.
In the not so distant future, this becomes less of an issue as the technology improves. The more applications that you can use things like 3D printing and NC cutting for, the less investment required for the additional sizes because you'll just need a different program and no additional equipment. Doesn't really help you much now, but things are moving that way.