From my own personal experience, I'd say that there are layers to this.
I'd vote first for Hard Work. If we don't work hard and have the experience necessary, then we can't do the job. (At least in my opinion.)
I'd vote second for Who You Know. 80% of the jobs I've gotten have been because I've known someone, or had some sort of friend-of-a-friend connection to get my foot in the door. Or simply because someone gave someone else my name, and I didn't even know about it.
And third, I'd vote for Luck. Based on Hard Work and Who You Know, Lady Luck - in the case of right place, right time - has seriously been on my side for at least 90% of my jobs.
Because I'm a hard worker and have made and kept connections with various people in various positions where I have worked along the way, I just happen to be available for great jobs when they come along. A woman I haven't worked for since 2008 pops back into my life for a brief stint to ask me a few questions about Stage Managing in the summer of 2012. She comes back again in April 2013 asking if I know anyone available for a summer gig with her festival. I said I might be. By November 2013 I have a signed LOA to be a Production Manager for her festival. She knows I'm a hard worker, I didn't apply for the job - she was someone I knew and worked for in the past, and, as luck would have it, I'm completely available for the position when she needs me. I've had several opportunities just call me up when I was in need of a job - so I consider a lot of it luck. My reputation (or at least what others say about me) may precede me, and perhaps that falls back to the Hard Work category and even Who You Know, but for me - once you have those two, sometimes it all boils down to luck.
At least in my opinion, Luck has brought me my career for the past 11 years.