I can't say why this market stays decentralized for certain, but its size and lack of geographical boundaries must be defining factors as to why it has remained this way. It is my understanding that the very fact it is not centralized is what makes this market so great from a traders perspective, in the sense that it is as close to a "fair market" as there can be, as traders are the ones who determine what the market value of any given currency is.
This of course not withstanding interventions by central banks as Pharaoh has mentioned, but even these interventions need to be supported by traders or they are short lived. Take for example the Yen weakness which occurred 2012/2013. This intervention had been talked about for sometime and the fact that BOJ was talking, in turn had traders talking and overall market sentiment moved to weaken the yen and trend traders have had a heyday.
In my opinion, lets keep it decentralized.