Because the purpose of the promotion is to get new and occasional sellers to list more and sell more. The fees on these sales is slightly higher than we Store owners pay.
However, if you really are trying to make money for your cancer treatments (sorry to hear that is not covered by universal medicare in Quebec, unlike here in Ontario), you should be selling as a charity and registering as such with eBay. Sob stories are generally a turnoff and you are probably losing sales by talking more about your illness than about your product.
Generally, it is not worthwhile selling anything on eBay for less than $10 unless most of your sales are multiples of your items.