It boils down to power, and lack of market demand.
There are specialty cooled-sensor cameras out there. They're generally just used for astrophotography.
The cooler that is used in almost all cooled cameras is what is called a thermoelectric cooler, commonly colloquially called a "Peltier" or "Seebeck cooler".
Generally, you will need a fairly chunky peltier to keep a image sensor cooled. For example, the Orion StarShoot G3 draws 12V at 1A to keep a 1/3" image sensor cooled to -10°C. That's 12 watts!
To calculate battery size needed, you multiply the current draw times the running time. As such, you would need a 1 Ah, 12V battery to run a cooled sensor for just one hour. As a comparison, the common Canon LP-E6 battery (as used in a Canon 5D2) is just 7.2V at 1.8Ah. Even ignoring the voltage difference, that's less then two hours of runtime with the camera on, for a much smaller sensor.
Furthermore, cooling a sensor is unlikely to do much to reduce ISO noise! Cooling a CCD/CMOS sensor largely reduces the dark current. However, the effects of dark current are purely a function of exposure time, so it only really helps with long exposures. High-ISO exposure noise is as much or more of a function of the CCD/CMOS sensor read noise then the dark-current noise of the sensor.
Readout-Noise is not affected by cooling the sensor, so high-ISO levels will be noisy, even with a cooled sensor.
Basically, there is really no reason to bother cooling a image sensor other then long-exposure. It offers very few benefits, and requires considerable additional system complexity, and massively increased power draw. A cooled system has to run continuously for the duration in which one expects to take photographs, as the cooling system will likely take many minutes (10-30) to cool the sensor down, and for the temperature to stabilize.
Furthermore, thermoelectric coolers are highly inefficent, and dissipate all the transfered thermal energy as heat. As such, a 5W peltier will dissipate 5W + any energy removed from the image sensor. This will almost certainly require active cooling, as the cooling efficiency is directly related to how cool the "hot" side of the peltier is.
It's actually common for high-end cooled image sensors to use liquid cooling, and they can dissipate many tens or hundreds of watts of heat.