The Mig 15 was designed from the outset to sweep the skies of the Soviet Union of any US A-bomb carrying aircraft. The suitability of the three cannon armament arrangement in this role was proven when the Migs forced the USAF to cease daytime operations over Korea with the B29.
The prime reason for the US retaining the .50 calibre well past its sell by date was cost. They retained huge numbers of this weapon and its associated ammunition in inventory after the end of the second world war. In the post war period they had no desire to fund starting production of a new weapon when they already held vast stocks of weapons already paid for
In terms of weight of fire, four 20mm cannon outguns six 50 calibre by a significant margin.
For the same reason the UK choose to arm the Hunter and Javelin with four 30mm Aden guns. This new generation was designed to protect the UK from A-bomb carrying Soviet jets. As it was considered that any defending aircraft may only have a fleeting opportunity for one burst of fire, a high volume of fire was of prime importance.