IMO... With companies, regardless of the industry, trying to run "lean" and show great quarterly numbers to shareholders... Parts inventory overhead costs don't look great on P&L statements...Especially in this economy with the recent earnings history that has just been overcome.
Times are tight, shareholders want to see a return on their investment (and are entitled to it according to SEC regulations).
The dealers are already stretched pretty thin and have to invest in this overhead out of pocket or take out loans for inventory and pay interest on it. Not being able to return it to the factory if they "guess' Wrong on the parts plays a role as well.
I agree with Kinzer, a dealer should have parts in stock like A-arms, voltage regulators, bumpers, handlebars, hand warmers, rivets, side panels etc... but that all costs money and some smaller dealers might not be able to stay in business with all that overhead.
I'm not trying to defend the back-order situation... just an observation as to "why" this might happen.
And the world turns.