Port to Port, which is the Cape to Cape's sister event, gets about a quarter of the turnout that this race gets. I think part of the reason behind it is the massive number of events - particularly the ones that weekend warriors do (obstacle and multisport races) - in the east so they give these a miss. We're kind of limited in the west.
There are exceptions - the Kowalski Classic recently got over 1000 entrants but it's the last time they're running it. That's a race I've spent my entire life training for ever since I heard about it 6-7 months ago and now I can't do it :-(
I'm keen to see how many people will shell out the $150-$350 entry fee for the Spartan race this year (not me) or do the multi-day option.