First, I want to say upfront that this question need not dissolve into arguments and discussion. This question can and should have a correct answer, please don't respond with your opinions.
GNUplot is very pervasive in Physics research. Many of the plots appearing in things like PRL and JPB are made in GNUplot. Why is this the case. There are much more modern tools for doing these types of graphs, and from my uninformed position, it appears that this would be easier.
One obvious and relevant first point to make is that GNUplot is free and opensource. I respect this, but do not expect that this is the primary reason.
I am hoping for answers that specify what GNUplot can do that can be achieved in other programs efficiently, say Mathematica. If there are none of these, and the reason is simply tradition/resistance to change, that is also fine, but I expect there must be some specific tasks one wishes to perform.