Ok here's what I think the reason;
Treatment of glaucoma is either decrease AH production or increase the outflow through the canal of schlemm
Beta blocker decrease production
M3 receptor agonist increase the outflow -- parasympathetic cause ciliary muscle contraction and relaxation of suspensory ligaments and so they aid in the outflow of the aqueous. Therefore, antimuscarinic are contraindicated.
As for alpha one receptors they are not involved in the ciliary muscle at all so the use of blocker or agonist is not going to work. However, agonists may be used to cause vasoconstriction and thereby decreasing the associated inconvenient congestion seen in open angle glaucoma.
This is just my thought of it and I have no reference