When I first started in ATC in the late 60's, there was a gentleman in charge of DCA (Dept of Civil Aviation) by the name of Sir Donald George Anderson. At this moment I'm looking at my first pilot's licence and ATC licence signed by none other than Sir D.G. Anderson.
As far as to the question of who decided on the question of what were the criterior as to authorising establishment of control towers, it was Sir D.G.Anderson. When he said "jump..." well you know the rest
Hence when I grew up in Madang and learnt to fly in Lae, both had ATC towers.
In the early 70's when jets started to operate RPT services in Australia (F28's) the norm was ATC required.
Time moves on.