There were two main reasons why the Tea Act reignited colonial resistance.
First, there was the issue of the tax on the tea. True, the tax was a very small one and tea would have remained cheap. However, it was another example of Parliament imposing a tax on colonists and they resented that.
Second, the Tea Act was a threat to American businesses. The East India Company was allowed to simply sell directly to retailers. This cut out the colonial wholesalers who had made money off the tea trade in the past. These merchants were very unhappy and worked to convince others that the British government might later do this sort of thing to merchants who traded in something other than tea.
In these ways, the Tea Act reignited colonial resistance. It annoyed colonists because it was a tax and it frightened colonial merchants because it made them feel that the British government might take away their livelihoods by giving monopolies to British firms.