Because it's generic term and it can be used in a subversive manner. The enemy always seeks to subvert important meanings by removing their definition. Satan is a very definitive way of looking at the generic notion of "God", it's related to Truth, Immortality, ever-existence etc.
Satan is definitely GOD, with the proper meaning and a higher meaning than even xians understand. Also, Satan should be understood as being higher in comparisson to being merely "a" God or simply "God", which is a word commonly referred to the all encompassing energy and consciousness of the universe.
There is a lot of confusion and enemy control around the word "God". Even the word "God" in itself is corrupted, the real word for what people mean as God being "Demon", from Ancient Greek terminology.
To avoid conflation and be specific, we call our God Satan, which is direct and unalterable. It also has all the underlying definitions that do not allow corruption to ensue.