I can think of 3 reasons:
1. Regulation: The amount of manouvering during the initial stages of a SID is limited by procedure design recommendations to reduce workload & errors during that critical transition phase.
2. Performance based obstacle clearance: If you are already at a maximum performance airspeed, climb rate (and therefore gradient) reduces as bank angle increases.
3. Navigation based obstacle clearance: Turn radius (and therefore ground path radius) reduces as angle of bank increases.