Little near surface hydrogen, thats why.
Hydrogen has a much larger cross-section than Carbon. As a result, CO2 ice wouldn't necessarily scatter neutrons more than it's surroundings. I don't feel like digging through the CRC to see if H20 and CO2 are listed somewhere in there but I'm going to go out on a limb and say at LEAST 10 times....
To SEE Carbon, they're going to have to look for distinctive Carbon-nuclei gamma ray photons with the GRS.
It looks like the southern highlands are a nice little cold-trap for water (ice/vapor/gas). Is this a seasonal thing? Anyone know if it's southern winter on Mars?