Because if we didn’t we’d go to jail. Okay, but there are other reasons why taxes are due by that particular date.

The United States was founded after declared independence from the United Kingdom, an action in part due to “taxation without representation.” Not being taxed was an American concept that lasted for a good 140 years…until Congress passed the 16^th Amendment in 1913, establishing an income tax. At the time, the rates ranged from 1 percent (for those who earned under $3,000) all the way up to 6 percent for those with the highest incomes. In early 1914, Congress declared that March 1, 1914 would be the first tax day—the date upon which taxes for the previous calendar year were due to the Internal Revenue Service. That date was chosen because it was about a year after the ratification of the 16^th Amendment.
The IRS quickly realized that it was a tremendous amount of work to process the tax returns of every American adult (or at least those who had paid), so in 1918 it moved Tax Day back two weeks to March 15 to give itself more time. The current tax deadline of April 15 was established in 1955, again for more time. Then again, time is money. By having a filing date that late into the year, the federal government is able to earn interest by holding onto previously withheld payroll taxes before issuing refunds.
It’s not always on April 15, however. If that date falls on a weekend, it’s moved up to the following Monday. This year, Tax Day is April 18, because April 15 falls on a Friday. In Washington, D.C., that day is a holiday: Emancipation Day, commemorating the 1862 abolishment of slavery in the district. Residents of Maine and Massachusetts get to wait until April 19, because April 18 is Patriot’s Day, recognizing the Revolutionary War battles of Lexington and Concord.