While Mars' atmosphere is mostly CO2, a greenhouse gas, there isn't enough atmosphere to create an effective insulating layer to hold in the trapped IR radiation.
Explanation:
The remaining gases are a mixture of nitrogen, argon, oxygen and carbon monoxide. CO2 is a potent greenhouse gas, so Mars does have a greenhouse effect. But it's very weak because the Martian atmosphere is so thin -- 100 times less dense than the Earth's atmosphere.
