In my experience this is (perhaps crassly) what's most important on a falling scale:
1. Your success and how "big" you are. You have to understand that it's not just show business in front of the camera, it's behind the camera as well. Ad agencies/clients/financiers want to be able to say on Monday over coffee with their partners that they were working with Roger Deakins or an Oscar winning DP. Coolness by association. I know for a fact, because I had it from his agent himself, that commercial clients will wait or work around Emmanuel Lubezki's schedule. When he's off a feature film all he needs to do is call up his agent and say "I have 2 months off, can you book some commercials?" and they book as many or as few as he wants. But understand that this only happens at the very top level. No client or ad agency will wait for us lesser DP's.
2. Recommendations and reputation. Directors will base hiring on sometimes unresearched recommendations from other directors or producers. Sometimes they might ask around about you, or someone else mentions you to them, or your name has a buzz at that moment. This is very common and once you have been vetted by their peers, they'll hire you based on that and not the particulars on your reel. It is very, very common that if you do a successful or good ad at a certain production company (and you and the director get along), you will pick up other work from other directors at that production company. They talk about DP's all the time.
3. Reel/CV. And here we have different tiers. Obviously a bigger reel (notice I didn't say better), with more things they recognise or have seen, with known actors/stars, stand a better chance than the ever so nicely shot thing they don't know about or haven't heard of. They need the stamp of approval of familiarity, or success, to be able feel confident in your skills. It's the sad truth that a beautiful reel on its own will not get necessarily get you more work. However, it will get you those first jobs that then can help build your reel.
Being a DP is great. But it's a long hard way to make it in to a successful career. It's just like life - it's not going to be fair, it's not going to be equal. I've had focus pullers that used to work for me and assistants that used to load my camera truck (at 4am in the morning) who today are shooting big Hollywood features and are much more successful than I am. Some of them got an easy ride and found success very quick by being at the right spot at the right time. That's OK, you just have to stop comparing yourself to others and not feel intimidated by the fact that it's not always fair. It doesn't mean you're a worse DP or have lesser skills. That's very important to remember.