Getting to Mars takes a really long time, about 9 months using today's rocket technology. This is because regular rocket engines burn fuel and oxygen together (like a car engine), but they're not very efficient. The fundamental problem is that spacecraft must carry both fuel and oxidiser since there's no air in space to support combustion. This creates a vicious circle: the more fuel you carry to go faster, the heavier your spacecraft becomes, requiring even more fuel to accelerate that extra weight. To go faster, you'd need massive amounts of fuel, making the rockets incredibly expensive and heavy. Current chemical propulsion systems are just about at their theoretical limits, with little room for improvement in efficiency.