I already did that. It's the same reason why in a market regulated strictly by consumption i.e. a free market there can be no racists businesses because they'd drive themselves out of business by discriminating. Same goes for miners who would discriminate against those paying less fees than such a miner would demand.. that "business" wouldn't simply be gobbled up by some other miner and so the competition for putting as many transactions into a block paying as much as possible would force all miners to accept ever decreasing transactions until we Bitcoin ends up with only a handful of miners providing not nearly enough security.