Because it's obvious...
Let's see it this way. Ice/Grass.
3=Super Effective
2=Normal
1=Not very effective
Abomasnow vs. Milotic
Milotic used Ice Beam!
Abomasnow's type Ice will defend against that so Ice Beam is 1, but its other type, Grass, is weak against Ice, so Ice Beam is 3. What's the average? 2 is the average, so it will do normal damage.