Here’s why: the dirtiest moments in the internal combustion engine’s operation are when it is starting.
The more times that the internal combustion engine in a hybrid turns off and the restarts, the more times you’re running that engine through its dirtiest phase of operation. On ordinary internal combustion vehicles, a catalytic converter ensures that the emissions components of the initial start exhaust are cleaned up. Hybrids, however, don’t use catalytic converters.
For the full story:
http://www.topspeed.com/cars/car-news/-hybrids-don-t-save-much-gas-or-emissions-ar52818.html