Gaining and losing orbital energy are both equally hard in the vacuum of space, it's not harder to get from Earth's orbit around the Sun to Mars's or vice-versa - though it takes somewhat different amounts of delta-v to go to an Earth-Mars intercept from them. The reason it's easier for ejecta to get from Mars to Earth than vice versa is that Mars has a shallower gravity well to climb out of before making the inter-planetary trip. It's a lot easier for an impact to impart the ~6 km/s velocity you need to get from the surface of Mars to hitting Earth's atmosphere than the ~13 km/s it takes to get from Earth's surface to hitting Mars.