Not sure it’s already been mentioned, but another reason to use rats is their lifespan. An average lifespan for a rat is right around 2 years. Two years is still practical enough to administer a drug on a daily basis and look for oncongenesis near the end of the rat’s life. You then can have a pretty decent idea (after examining all of the data) on whether or not a human may develop cancer if they were to take the same drug for a majority of their life. You wouldn’t do such a study in non-human primates because it’s completely impractical to administer a drug daily for decades at a time.
Latest Answers