Range,
A string loop is about half the diameter of the average served string, and more important is the fact that it is limp and flexible. The peg digs in hard to the loop, and the non-torque flexible nature is what keeps it there. If you rotate your hand in either direction (which will also rotate the peg, the string loop will rotate with it.
The larger, inflexible nature of the taut bowstring will make it very dangerous if you rotate the head of that release at all during or while at full draw. It could slip right off.
That and the tight bowstring wouldn't really seat as well as a string loop......you could easily not be into the deepest part of the peg to begin with as you attemtp to draw.
IF you could draw straight back like a machine and anchor level everytime you could probably do it but I wouldn't recommend it.