Because the incoming legal tea from the East India Co. was suddenly cheaper than the tea smuggled in by folks like John Hancock--yeah, that John Hancock, a ship owner.
And, the incoming tea threatened to undermine the cost and profitability of the smugglers.
It is an amazing story, but the deeper lesson is how much hype and nobility we've been fed about more than few Founders and men like Lincoln.
For example, when you clear away all the adulation, you find that many of the Framers of the constitution at the constitutional convention were politicians and lawyers. And, very many of them ended up in the new national *ahem* federal government. Oh, like maybe now we can start to understand why the constitution has a few loopholes in it, such as why the door wasn't closed on implied powers.
And, wipe away the adulation and suddenly you can understand why the Federalists were oh-so willing to violate the First Amendment of the constitution they argued and toiled so strenuously for--the Alien and Sedition Acts. Its because plenty of them were politicians.
And, wipe away the parchment idolatry, and you can see that there were sneaky dealings at the constitutional convention. A glaring example is that the pro-constitution crowd called themselves Federalists. Ha! That was public-relations image genius! Why? Because the states already had a federal union under the Articles of Confederation. At that time, the words federation and confederation were synonymous. But, by calling themselves Federalists, they forced the anti-constitution crowd to be called Anti-federalists, giving the connotation that they were opposed to a genuine federation of independent states.
The real lessons are not in our typical grade-school history books. It all comes down to power and money. Money gets and supports the power. Power needs money. Power is what too many of them want.