1.) They fish when the conditions are right.
When people ask me if they should go fishing tomorrow, my response is always, "yes". I think that unless the weather is unsafe, people should go fishing whenever they get a chance. However, many of the most successful anglers go fishing when the conditions are right for the fish to cooperate. Now, this could mean that they pick their days or (most likely) they choose to fish during the part of the day that the spots they plan on fishing will produce. Tides (in saltwater), wind speed and direction, time of day, time of year and many other factors determine when the best times to fish certain spots are.
The most successful anglers plan their fishing trips around the factors mentioned above. They don't have "favorite spots" that they hit every time go fishing. They have spots that are their favorites only at certain times and they fish those spots only when the conditions are right. Understanding how these factors affect the areas you fish and knowing how to take advantage of that knowledge will put you closer to be in the "10%".
2.) They are properly equipped to meet the challenges of their fishing trips.
Being properly equipped has many aspects to it and if one link in this chain breaks, you're not going to catch many fish. We'll start from the beginning. Reliable transportation is a must. If you're vehicle doesn't get you to where your fishing excursion begins, you're not going to catch fish. If you're a kayaker and your kayak isn't properly maintained and doesn't perform well during the trip, you aren't going to catch much. If you only take one fishing rod/reel with you and it is of low quality or is poorly maintained, it can fail on you and you're not going to catch much that way either.
Let's say you have all that stuff under control. You keep your vehicle, kayak, tackle all in top notch working order. Now, you have to have the right bait and know how to use it. Successful anglers know, or have a good idea, what type of bait they will need for each particular fishing trip. They will do whatever it takes to get their hands on it and they know what to do with it once they get it. Whether it's live bait or lures, the correct bait choice can make all the difference in the world. Also, on the rare occasion that successful angler aren't sure what to use, they bring everything and the kitchen sink. For example, just today, a good friend of mine, who I would say is in the "10%", knew that the fish can be a little finicky where he was fishing today. So he picked up some baits that he hadn't used in over 6 months just in case. He started the day using what every other fishing guide within 50 miles had been using for months but the fish weren't eating it. He broke out the "other" bait and proceeded to have his clients catch 50+ redfish.
Being properly equipped will raise your chance of sliding over to the "10%" column for sure.
3.) They are knowledgeable and experienced.
Everybody starts out in the "90%" category, I don't care who you are. How long it takes to, or if you ever, transition from the 90% column to the 10% column is determined by how well you learn about fishing in your home waters and the sport in general. The two previous points are only attainable through this third point. Your knowledge and experience of your home waters, how the fish behave there, and how to be properly equipped for fishing there will help you determine when to fish where and how to be equipped for success.
How passionate you are about learning these things and your ability to put them into practice will be the determining factor as to which column of success you are in. The 10% or the 90%. Read magazines, watch fishing shows, join a fishing clubs, surround yourself with successful anglers...Most importantly GO FISHING. The more you fish the more you will learn. Experience is invaluable and you can't buy it. Don't be frustrated by failure. There is no such thing as an unproductive fishing trip. You should be able to learn what doesn't work on even your worst days and simply as a matter of the process of deduction you will eventually learn what does works.
These are just 3 reasons that 10% of the fisherman catch 90% of the fish. There are many, many, more smaller reasons but these are the big ones. If you master these, you will soon find your self as a "10 percenter".