Air support could be limited. If the Covenant control the skies, it's understandable to not have much air support, but it is not unheard of to make exceptions and prioritization for units based on the importance of their mission. If there is any degree of air power available, - and I assume there is considering you are being dropped in pods - it'd be special units like yours who would be getting air support. 

Mars is very important to the war effort. There has to be some kind of support available.