It isn't entirely clear, however the most likely explanation is that it was because the principal royal residence during the Tudor era was the Palace of Placentia in East Greenwich (East Greenwich is modern-day Greenwich, West Greenwich is modern-day Deptford).

Remember that lands would be granted subject to feudal tenure. As such, whenever the Tudors granted lands to people, they granted them as part of "the royal Manor of East Greenwich in the county of Kent" by default because that was where they were based. Eventually, this became a custom whenever the English crown granted lands, and by the time the colonial charters were drawn up things were done in the same way:

There is no means of discovering with certainty the reason for the choice of East Greenwich by the commissioners for the sale of lands or the officers who drew up grants in the early years of Edward VI. No order of the Court of Augmentations or of any other authority has been found giving definite instructions on the point. But there can be little doubt that Greenwich was settled upon because it was the most usual dwelling-place of the king at that time. Where the king was the court was, and necessarily there fore the officials who had charge of the sale of lands. It might be anticipated that Westminster should have chosen, as it also was an occasional residence of the king, and the permanent location of the law-courts and of a considerable body of clerks; but apart from the less frequent stay of the court there, Westminster was not a manor, and therefore was not available.

[...]

James I. preferred other royal dwellings, and with his accession Greenwich, like much more that belonged to the sixteenth century, passed under a cloud. But this was no longer important from the point of view of the form of crown grants. Both at the time of the creation of the custom and during the period when it was becoming fixed, Greenwich was in every way the most natural place to be chosen as a source of tenure, and by the time of the Stuarts the custom had obtained the same rigidity as other legal formulas. Its use in the charters given to the colonies by James and his suc- cessors is thus quite explicable. But after all it had little if any real significance for the colonies. It was simply an adaptation to land beyond the sea of a form originally used in the grant of crown lands in England.

Source: "The Manor of East Greenwich in the County of Kent", Edward P. Cheyney, 1905