the reason why it wouldn't is this:
the majority of people still believe weed should cost as much as it does.
there is still a lot of greed in this world.
and when you don't have perfect competition, ONE person cannot change the entire economy. in perfect competition, one business could change the prices within a 10 mile radius.
for this to happen in the black market/ decriminalized system, you have to have the majority of dealers within the area believe in their price of weed, outsell the competition.
but either way, the majority of these problems with dealers is greed. as crackerjax has said.
because even if they realize that weed can be grown outside very cheaply, and that it might be fair to pass these savings along to the consumer. their greed makes them think, nah, why do that when I can have over 200% personal profits?
that's what's wrong with the system as it is.