The short answer: Too many things and it's probably too individualized for a blanket answer, but the nature of life makes us all very diverse in myriad ways to maximize human survival in the long run.
The long answer: While our current scientific understanding of human ticklishness is not complete, we do know some factors that do contribute to it, and there are many. The sensory nerves of the body have more nerve ending concentrations in some people and less in others. There are a ton of biochemical states in the body that can alter how nerves function and how the brain perceives the received signals from them. The things that affect those processes are even more numerous, from our activity level, nutrition, and age-related hormonal states, all the way to how much sunlight we perceive with our eyes or that contacts our skin in a given day. Probably one of the biggest factors is the part of the brain that processes most of the tickling response is also the part that deals with emotions and various other processes. Everyone has a very unique fingerprint for exactly how they process and perceive emotions and that in turn can be influenced by so many things, to include many of the aforementioned ones, genetics, and one's experiences in development.
You have asked a simple question which would require an almost impossibly complex answer. Scientists study this a lot because it gives a lot of insight to a lot of the body's processes, and especially insights into the brain and how it develops perception.
On another note, I probably should have been a tickle scientist. They didn't teach us about that career path when I was in high school...