The reason behind it is partisan. They want to support the notion that the Union bears no blame for triggering the war, and it was all those hot head Confederates who attacked Sumter for no reason whatsoever.
Calling it "supply ships" supports this fiction. Calling it what it was, a belligerent War fleet with orders to attack them, calls into question the narrative that the South started it.
They don't want to confuse people with facts that cloud the narrative.
While you, on the other hand, want to promote the myth that it was all Lincoln's fault and the South was blameless. So where does that leave us?