I don't entirely agree with some of the comments here -- my mother-in-law, a Thai farmer, has visited us twice in Canada. The first time, she had no travel history, and was given a 6 month tourist visa, no problem; we applied for her to accompany my wife when she landed. The second time was when my wife was pregnant, and we wanted her to come here to help take care of my wife and the baby; she was given a 3-year multiple entry visa. In the latter application, we were explicit about why she was visiting -- her daughter's pregnancy is an obvious reason, and even CIC isn't silly enough to think that a mother is going to come visit and not do things like watch the baby, change its diaper, take care of the older child, etc. Come on!
I have found CIC to be extremely flexible regarding visits by relatives -- personally, I would guess that your refusal is due to two possible things, not enough assets on your end, and maybe an incomplete letter explaining your family situation. It might be optimism, but I have always found that CIC is quite accommodating to things that make sense, and if you take enough time to think about how they might look from the other side and explain, most things are possible.
N.B. I'm not arguing that there aren't any absurd visa refusals, of course. For the man above, with a Chinese mother-in-law, I have no idea what happened, your situation sounds similar to mine.