First, the technology within the AA and AAA lithium 1.5-volt batteries is geared towards devices with a heavy demand for energy and are generally used for an extended period of time (like MP3, Digital Cameras and handheld games). The most common applications that use C and D batteries do not have the same power demand profile. Secondly, the AA and AAA make up more than 60% of the overall market demand for traditional alkaline sizes (AA, AAA, C, D and 9V). The AA and AAA go into more devices, which means more sales opportunities (and more profit) for the manufacturer. The manufacturer will not make batteries if there are no devices for them.
And finally, cost. A 4-pack of the lithium 1.5V AA is up to 3X as expensive as traditional alkaline. Most people are not willing to pay $6 for a 1.5-volt D cell to power their flashlight or kids toy when they could buy 4-alkaline D cells for the same price. At this point it is cost prohibitive to have a C or D 1.5-volt battery.
One side note: there are 3.0-volt and 3.6-volt lithium C and D batteries available. Unfortunately, that voltage is not compatible with devices that require 1.5-volts out of the C or D battery.
