Because I've got burnt by Django in the past. If you do some search on the HN and elsewhere on the difference between two, the common answer that you'll find is that they are similar and it doesn't matter which one do you pick (given that you are not proficient with any of them). I don't agree with that premise and decided to make a case here.
So if you are ever in a position to pick between Django and Rails for CRUD-y SaaS app, go with Rails. You can't go wrong with that.