A couple of reasons with the main one being that I wanted it. The 6.5x57 was more in tune with my preference for the x57 case and would be a good companion for my 7x57. Cases were another consideration as I can make cases from any of the x57 or 06 based cases even though I got a bag of headstamped 6.5x57 cases. Getting a set of Dies for almost givaway price didn't hurt its desirability either.
In the end, when it came down to it, I saw this rifle as being a bit special for me given that the action and blank came from friends so it deserved something a little out of the ordinary run of cartridges to compliment it and for the type of hunting I do now the 6.5x57 is right up there in the multipurpose capability. I know the x55 would do the same thing but then so would a host of other cartridges which I also discounted as I have a bit of a fixation on the x57 case with it being the grandaddy of all the modern cartridges and about perfect for a multitude of bore sizes and bullet weights while the 6.5x55 is a bit of an orphan with nothing outside of its primary designation ever having been done with it.