There are some medical conditions that, if they just are really terrible or costly to treat, will be grounds for refusal. Even if they are your parents or your spouse or whomever, they can still be denied. For better or for worse, that is how it works. Angelbrat was just saying the facts. Plus, she gave you good advice. She wrote "If you can successfully prove that neither parent will be a financial burden on Canada because of their health, then send in the proof."
In regards to "Doesn't matter if they have medical condition or not the expenses towards them cannot be zero compare to other average Canadian." -- I'm not sure what you meant to say, but the "average" Canadian has contributed somewhat in the form of taxes to health care while a permanent resident who is a sick elderly parent has not done the same.
It's not to say I completely unsympathetic to your situation. It is unfortunate that your parents may not be able to join you in Canada! If you are close to your parents and genuinely care about their well-being, I understand that it could be hard to have them so far away and to worry about them
What did CIC actually write in the letter? They simply said that they are refusing them "due to medical inadmissibility"? Nothing more? Did you submit all the tests needed? Perhaps they have a condition that makes them inadmissible to enter Canada? If it's not a condition they have, perhaps you are lacking documentation?
Or, now that I think of it, as many family sponsorship for parents take a few years, perhaps they are requesting updated medical tests? It may be their way of giving you a chance in case your parents' conditions has improved since the application date.
Additionally, I cannot give you my personal experience on this matter. I can only provide you with some token advice.