Because that's how the English authorities divided up the colonies nearly 400 years ago.

Under a 1609 grant to the London Company, Virginia consisted of the North American coast stretching 200 miles north and south of "Cape Comfort" (now known as Old Point Comfort in Hampton, Virginia, which is the site of Ft. Monroe) and the lands inland to the Pacific Ocean. This includes every square inch of what is now Maryland and, obviously, the entire Eastern Shore or Delmarva Peninsula. Although Virginia was huge, early settlement was focused on the banks of the James and York Rivers, the Virginia Peninsula lying between those rivers, and the southern end of the Eastern Shore. The capital at this time was Jamestown, which is on the north shore of the James River.

So to my mind, the question really is, "How did Maryland come to be where it is, including part of the Eastern Shore?" And I think that is the right way to frame the historical question, even though I'll admit to having been born in Virginia and thus having a bit of a bias.

Why does Maryland even exist? George Calvert, 1st Baron Baltimore, was an English nobleman and minister of James I who was very interested in colonial settlement. Calvert had sponsored a somewhat successful colony in Newfoundland's Avalon Peninsula. After announcing his conversion to Catholicism in 1625, which required him to resign his royal offices but didn't end his influence, Calvert went to the Avalon colony himself. He came to believe the climate was not good--that's Newfoundland for you--and sought another colony somewhere more hospitable; i.e., Virginia. Baltimore visited Virginia in 1629. The colonial leaders in Jamestown were rightly suspicious that he intended to wheedle their best land away. Calvert indeed returned to England and began lobbying for a grant of land in Virginia on which to settle Catholics. That is how he spent 1630-32.

Calvert initially wanted the land between the James and Chowan Rivers. That would basically be today's South Hampton Roads area of Virginia and Northern Inner Banks area of North Carolina, including what are now Norfolk and Virginia Beach, Virginia and Elizabeth City, North Carolina. (Another way to think of this is the Virginia Beach-Norfolk-Newport News, VA-NC Metropolitan Statistical Area less the portion north of the James River.) The Virginians argued this area and indeed the entire southern Chesapeake Bay was firmly within their domain and either was settled or imminently would be. Instead, they suggested that Calvert be given land further north in Virginia. This was of particular interest since the Dutch were beginning to settle the Delaware Valley, which was in the space between the English colonies of Virginia and New England. The Catholics would thus check Dutch expansion. Calvert agreed to this and may have believed he was going to be granted the entire Eastern Shore as well as the northern part of the Chesapeake Bay shore. He died, however, in 1632, before the boundaries of the grant were finalized. The Virginians managed to show that they had already settled the southern Eastern Shore, so it was excluded from the grant to Calvert.

The grant was actually made by Charles I to Cecil Calvert, 2d Baron Baltimore, who was George Calvert's son. This became the Maryland colony. It basically included an area on the North American mainland north of the Potomac River including the northern coast of the Chesapeake Bay and an area on the Eastern Shore defined as follows:

by a Right Line drawn from the Promontory, or Head-Land, called Watkin's Point, situate upon the Bay aforesaid, near the river Wigloo, on the West, unto the main Ocean on the East

Watkins Point (the apostrophe has been dropped), a headland projecting into the Chesapeake Bay ("the Bay aforesaid"), had been named in 1608 by John Smith for one of his expedition's soldiers. The Wigloo River is now called the Wicomico River and is not to be confused with another river of the same name on the other side of the Chesapeake Bay.

So the Eastern Shore south of the line from Watkins Point to the Atlantic Ocean was to remain in Virginia. Indeed, in 1634 the Virginia House of Burgesses (colonial legislature) organized the Virginia portion of the Eastern Shore as Accomac Shire. (Yes, Virginia's county-level local government units were originally called shires.) It was renamed Northampton County in 1642. In 1663, Northampton County was divided into Northampton and Accomac Counties and so they remain, with the exception that a variant spelling of Accomack became official for the county (though not its principal town) in 1940.

Tracing the line from Watkins Point to the Atlantic was actually somewhat difficult. Initially, it didn't matter, since no colonists lived in the border region and the colonies had other problems, like being in a state of quasi-war. Virginians who objected to the very existence of Maryland had tried to crush it in the cradle by settling Kent Island, which is very close to Annapolis. There were armed clashes to evict them. There was also fighting in the Chesapeake Bay. This mostly wrapped up by the 1650s.

The issue of the Eastern Shore border became important in the 1660s because some people, especially Puritan and Quaker refugees from England, were settling there to avoid notice and taxation. The colonial authorities now had a common enemy. In 1668, the Maryland and Virginia authorities agreed provisionally to set the border at 38 degrees north longitude, based on where the Potomac empties into the Chesapeake Bay on the other side. This is actually north of Watkins Point and also creates a nearly inaccessible marshy part of Virginia on the north shore of Pocomoke Bay west of the Pocomoke River.

In 1688, a proper survey was to be conducted, but the surveyors decided rather than wade through the marshes to simply give Maryland the north shore of Pocomoke Bay (including Watkins Point) without really surveying and proceed up the Pocomoke River to the 38th parallel, then properly survey a line east to the Atlantic. But they used magnetic compasses so they were actually heading about 5 degrees north of east. This accounts for the discrepancy between the modern Eastern Shore border and the one in the 1632 charter as well as the angle of the modern boundary line.

There were further uncertainties about the border across the Chesapeake Bay, which became important for the oyster industry. This led to some interesting negotiations and clashes in the mid-19th century, in the course of which Virginia argued for a time that Watkins Point was actually much further north!

Anyway, when the colonies subsequently declared themselves independent states and formed the United States, they largely didn't change their borders. At the new country's supposed birth, the Eastern Shore had been split between Virginia and Maryland for over 150 years and the border had been surveyed for over 100. Changing it would have created chaos and there is no evidence anyone seriously contemplated it.