Why wasn't Maine an original colony?

Maine itself was not a separate colony because it was not granted a royal charter. Instead, the royal charter for the Massachusetts Bay Colony included some of the land area that would eventually become Maine. Maine continued to remain a part of Massachusetts until 1820, when it became its own state. The most likely explanation for not making Maine its own colony is that the area of Maine was contested by both the French and English until after the French and Indian War. By that time, Massachusetts had already governed part of the area of Maine, which was sparsely populated as its main source of wealth was logging for shipbuilding.
This is similar to the Virginia colony, which initially encompassed all of the land which later became West Virginia. When Virginia was granted statehood after the Revolutionary War, it included West Virginia, which did not become its own state until the Civil War, when West Virginia seceded from Virginia.