CCD's require more energy, thus run hotter - so high ISO performance is not their strength.
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As far as I know, sensor power consumption is irrelevant to noise except with continuously operated sensors (Live View or video cameras.) Where CMOS has an advantage, when used well, is doing on-pixel pre-amplification including possible adjustment for fixed pattern noise.
At high ISO speeds, an active CMOS sensor amplifies the weak signal at each photosite, and this amplified signal travels accross the sensor to the A/D unit. With a CCD instead, the weak high ISO speed signal from each photosite is passively transmitted across the sensor before being pre-amplified and converted to digital.
During the travel from photosite to A/D converter, the stronger CMOS signal is less prone to noise arising in transit. This seems to help good CMOS (and NMOS) sensors at high ISO speeds.
I agree about other factors like micro-lenses, absent on most MF CCDs (the 31MP Kodak has micro-lenses.)
There might also be design trade-offs reflecting different priorities: medium format is perhaps overall less interested in high ISO speed operation, and far more interested in getting the possible results under more controlled conditions, meaning mostly low to moderate ISO, using a tripod, studio lights, flash if necessary to avoid high ISO speeds. Dynamic range at low ISO speeds has traditionally been higher for the Full Frame Transfer type CCD's used in medium format than for CMOS sensors.
Finally, Michael R. has also suggested (if I recall correctly) that the costs of changing to CMOS is more justifiable with the higher sales volumes of smaller formats, while in the very low volume market for larger sensors, cost minimization favor staying with the existing full frame transfer CCD technology, at least up till now.
