Do many of you think its strange that in 40k Universe ground combat happen all the time ? Usually in advanced tech setting (star trek, star wars, bsg and so on) a battle is decided in space. Which to me makes sense. Once space superiority is established, hostile ground forces can be obliterated from space. The high cost, high maintenance Space Marines would be best suited to assault and take over spaceships instead of waste in attrition battle with orcs.
1) Fleets tend to be weeks away from the start of major battle operations. Initial operations tend to be done by local PDF regiments, and if planets are lucky, space marine chapters in the area or Imperial Guard Units in the area.
2) Battles tend to be done over crucial areas that cannot be risked exposure to orbital bombardment.
3) Setting up support spacecraft is time consuming. After initial conflict over space, they need several more weeks to slowboat support craft into friendly space.
4) They do use Space Marines for boarding actions. In the case of the game, the Space Marines needed to take out a high value target, the Ork Warboss, and secure objectives that the local Guard simply didn't have the manpower to do so. You never fight the bulk of the Ork army. The Guard are doing the dying to buy you time while you wreck havok.