Maybe the volume of pistols & ammunition required to make a significant difference would have made purchasing such a large quantity of them cost prohibitive.
When resources are scarce, decisions are made based on cost effectiveness and the bottom line.
Since the enemy didn't have them, it's not like Washington was trying to maintain parity (equality of arms).
I wonder to whom and where they would be issued, and would it have created a logistics problem to resupply them in the field with ammunition, parts etc...?
Maybe because they were short range weapons, they were given a low priority in relation to all of the other weapons, goods and payroll necessary to outfit an entire military war/operation.
In the overall scheme of things, how much of a difference would purchasing some of these have made, and how many more would have been needed to make any real difference?
I'm just speculating, but plowing money into one project usually means a reduction in funds somewhere else. So choices were made.
Maybe even who was manufacturering them and who was making the final decision or recommendation about buying them was the deciding factor.
If you were the President (or present) at the table along with the top military "brass" at the time, maybe you too would have readily agreed with the decision to not purchase any of them.