'No reason, its just a tradition!' Like many hoary military traditions, there was undoubtedly a reason, good or otherwise, at some point in the past and it is perpetuated now because it's traditional and because 'it distinguishes us from those other [common] regiments.'
And what MiniDef chooses to call it is not our concern!
Not too long ago I was part of military displayof WWI uniforms and kit. One of our 'props' was a wooden box containing an aluminium cook pot, insulated with straw to keep the food warm as it as carried up to the trenches. We call it a 'haybox'. The modern Militia [Territorial] types who saw it were absolutley gobsmacked because they call their plastic and metal, high tech insulated food containers 'hayboxes'
but have no idea why! Except, of course, 'That's what the RSM calls it!'