(1) Lack of demand for it, other than from a handful of folks on the internet.
(2) Pretty significant manpower commitment---on top of everything else involved in running an A-tier---especially if you're talking about enough marshals to truly officiate a tournament.
(3) Unless you have a surplus of very committed locals who will spend the entire weekend marshaling instead of playing or doing anything else, you're going to have to pay them. And any reasonable pay, for a staff of marshals for, say, 16 hours of tournament play, is going to add up to a lot.
(4) There are better things to do with that time, and money (if it comes to that).
(5) Discouraging people from wanting to run A-tiers, with such a mandate.
*
Again, TDs are free to do this, at A-tiers or anywhere else, if they think it's worthwhile. The PDGA's not stopping anyone.
+1 Agree on all counts. Some day when there are more resouces available to the top tournaments? Perhaps.