Arena tickets & skeleton keys are both hard to get. PvP is fun and skeleton keys are desired by all. This means that, if skeleton keys were purchasable with tickets, it would both provide users with a reason to play in tournaments (ultimately filling more brackets in tournaments) while also allowing them to know how close they are to a skeleton key. I feel like if you know how close you are to achieving something, it can provide a bigger sense of motivation than if you were to attempt to obtain something purely through luck.
For example, farming for alchemical extract is a lot more fun than farming for the uber gear as you have a decent idea as to how close to you are to a certain piece of gear.