Every year I seem to have at least one friend ask me to help with their taxes - typically someone who hasn't done theirs in 10 years or more, has no idea where to begin, and whose situation is getting worse every year.
So I always ask the simpletax people to add previous years, even offering to work for them or volunteer time, and the answer has been the same. A very prompt, polite statement that the tax code changes way too much every year and it is logistically impossible for them to cover past years.
I can only assume they're telling the truth and that the problem is impossible for them right now. But it deeply irritates me that the Canadian government doesn't issue its own (free!) software to make it possible for people with the best of intentions to comply with their laws.