Hi - I have a very good friend (someone I am so close - known her since I was a toddler- we even refer ourselves as sisters) who's boys both required kidney transplants - one was given by a family member and one by a friend of the family. Not telling you this to make you feel guilty but just to let you know going through some of this process I understand from a donor/friend/family perspective.
First as you have gone through almost to the end - with medical and others you understand that this is completely volunteer and no one should ever feel obligated to donate - it is so personal and such an individual decision that no one should feel obligated or to feel guilty about their decision. That being said I think you simply say you are so sorry but due to changes in your life you are not in a position to donate. You do not, nor should you, have to give a specific reason - it could be a medical reason or it could be like you said you have others that are dependent on you now to consider.
I would though give this news as soon as possible so they can reach out to get another appropriate donor. My friend for her second son - reached out very broad and got many, many people to go forward willing to donate - so many that I missed the boat and was put on a wait list. There are many generous people that are in a position that the risk to them is worth donating.