Because it matters,
As a noun, capital refers to (1) a city that serves as a center of government, (2) wealth in the form of money or property, and (3) a capital letter. As an adjective, it means (1) principal, (2) involving financial assets, and (3) deserving of the death penalty. There are other definitions of capital, but these are the most commonly used ones.
Capitol has two very specific definitions (outside ancient Rome): (1) a U.S. state legislature building, and (2) the U.S. Capitol building in Washington, D.C. State capitols are located in the capital cities of U.S. states, and the Capitol is located in the capital city of the U.S. If you’re not talking about any of these capitol buildings, then the word you want is probably capital.
The Capitol building located in Washington, D.C. is spelled with a capital C, but state capitol buildings ordinarily don’t have the capital C (which is not to say that some writers don’t capitalize them anyway).