We've been asking for this for a long time...
It's to avoid the situation where your jerseys closely matched the default GK jersey colours and you lose track of where your GK is on the pitch. It would be nice if it changed which GK jersey is used based on the colours of the opposing team. I had a match yesterday where the disc of the opposition GK was exactly the same colour as the discs representing my outfield players - good job I knew that I didn't have a No.9 on the pitch so could Identify the fool in my colours who was saving my AMs shots...