Logistics. The British had decided in 1938 that heavier guns were going to be needed, but that going to HMGs was not enough of an increase and would, at best, be a stopgap measure. They decided to skip over the HMGs and go straight to 20mm cannons.
Ahhh, I was talkin more like the set of 20mm Cannon and the 4 12.7mm. That would have been just ridiculous lol!