Because it would have been sold off by a previous government. (see virtually every other money making asset.)
Thats the point, its not trivial to sell it off, because it was set up to counteract a fooling king in exchange for a stipend.
This system was explicitly designed to take an asset from the crown and place it in trust so that the king could get a steady income, ostensibly from assets that had unpredictable income.
Thus the crown estate was born, which has a bunch of rules, most notably about taking on debt.
(dartmoor is owned partly by the crown)