It really has to be reiterated that denying clearance is not necessarily as a result of conflicting traffic.
The ATC may have complex procedures that they need to go through to allow your airways clearance. There may be coordination, route requirements, local instructions, etc etc, that all need to be sorted before a clearance can be given. Not to mention systems issues with a rapidly aging ATC system.
Plus, they may not just be controlling your particular part of the world. They may be getting pumped in another section of their airspace. Plus, because of the staffing pressure that airservices is currently experiencing (due to funding pressure from industry and government), these areas of jurisdiction are getting ever larger.
I am led to believe that you can fly all the way from Archerfield to Bankstown talking to a single controller (excluding a few minutes with the Approach guys). That's a huge area to deal with, considering some of the aviation hotspots in that particular corridor.
Please don't see me as an ATC apologist. I just don't like people in this thread that are blaming ATC, when instead it's the system and culture created by industry that might be worth looking at.