Why do catalytic converters do not work effctively until the engine has warmed up?

A catalytic converter changes unburned hydrocarbons into carbon dioxide and water vapor. It's a simple chemical process, and like most chemical processes, it will either generate or absorb heat so in this case, since the catalytic converter requires heat to change the state of the hydrocarbons, it either has to be heated or it will operate quite effectively using the exhaust heat.

Note, if the engine is operating efficiently there should be very little for the catalytic converter to do.