Also other animals reproduce faster than humans. Generations of rats grow a lot faster than people. So if you wanted to examine stuff within the population, you would get faster results.
ETA: heh I thought of more!
-You can think of it as rats (or another animal) was born to be experimented on, kinda like a cow was born to be eaten.
- This is similar to the others, but risking health of animals is easier than risking the health of humans.
It is easier to fight for the other side... you can guilt people in to it.