The 18650 cell has a huge advantage in that it is used in a lot of other industrial applications (such as laptop computers) so all the R&D money goes into improving those cells.
The 26650 is less common and flashlights just don't create a large enough demand to spur the market into making more and better batteries at the same rate as the 18650. As a result, you have a pretty substantially larger battery with a comparably small increase in capacity. Plus, you don't get the same variety of cells with different balances of capacity and discharge rates appearing on the market. Thus, it doesn't make as much sense to produce lights for a market with much more limited battery choices.
One final thing, many of the 18650 lights out there will also accept lithium primary cells. That's adds even more flexibility to the form factor and will entice buyers who aren't ready to jump in to lithium ions yet.