Photoelectric smoke sensor
Carbon monoxide sensor
Heat sensor
Three activity sensors
Ambient light sensor
Humidity sensor

The fact is that the Nest Protect uses a set of sensors to accomplish the same thing (as an ionizing sensor). We have the photoelectric sensor which is good for smokey fires, we have a heat sensor which detects the hot flaming ones, we have a CO detector which aids in fire detection of both kinds, and we have a humidity sensor that we use to rule out false alarms from the pot of boiling water in your kitchen.

The problem with ionic sensors is they tend to sound false alarms, especially with humidity. They are actually banned in code from being within a certain distance of bathrooms for that reason in many places.

It is also true, as jcanavera said, that in Europe the disposal rules are more strict than the US. (Due to the radioactive material) That is certainly a factor as well.