First, we can note that both types helmets have essentially identical requirements for attenuating impacts (and nearly identical tests to validate their performance on crashes). So, there is no crash-safety reason for the difference. (Like, a bike helmet is not particularly designed to protect against an impact with a car.)

Thus, the main reason is due to different needs for ability to cool of ones head. A person driving a bicycle needs much better cooling (two reasons: typically warmer weather and the act of cycling itself). Thus, a bike helmet has plenty of cooling holes. The skiing helmet does not need these, thus, one can use a shell with far less ventilation (resulting in a helmet that is a bit more robust against wear and tear, in addition to a nice smoother look of helmet).

There are, however, helmets that can be used for both; and which are sold for both. These are clearly in between the fully smooth skiing helmet and the highly perforated bicycling helmet. A few holes, sometimes with adjustments. (Though, given the limited maximum ventilation, such helmets only suit leisurely cycling. A such, they are also excellent for winter cycling, especially if done at leisurely pace, say, to commute at a pace where one does not need to shower or even change clothes at destination.)