Because only consequences matter, almost any action could be morally right in a given circumstance. Must consider the indirect ramifications of an action because must consider the long-term consequences of the action. So, cannot solve the short-term dilemma without considering what the larger implications of the act will be. It is unworkable. Problem is when consequences are unclear, or when trying to compare levels or happiness or pain.
Some actions are wrong, even if they produce happiness.
Utilitarians have answer here. Again, this is question-begging. Also, the claim is that this would very rarely happen, and that in tricky cases, we could accede to other goods, such as promise-keeping, and honor them. It is unjust.