I can't remember corectly but I believe the taxes were funding a war against France, which is why the British were not able to supple enough troops to quell rebellion.
Yes, it was a war with France over the colonies, but Britain's colonies were the colonies Britain may have lost if the French had won. The colonists even helped in that war effort, and the British did still look down on them, however both British citizens and colonists had to pay taxes for the war, and the British had to pay an even greater one. Britain was in debt from that war, and Parliament's rationale was the the colonists should also accept some financial responsibility for their own defense if British citizens could pay an even greater financial tax. Anyway, I don't mean to single out any single group of people in history, of course. Few people like taxes and, depending on the time period and one's cultural upbringing, taxes may be seen as more evil than slavery. The rationale I see behind that, and I do not agree with the following (I'm just stating what one may justify slavery with) , is why should a person who is not a slave care about freeing a slave? Necessities and luxuries haven't always been as plentiful as they are in modern day, first world countries and people naturally do what is most beneficial for them when they don't have kindness to spare.