It happened right after WW2. American medium tanks retained the hull-mounted MG for a decade longer. While probably the most influential early design without one that participated in the war and became a mainstay of its weight class in the army was the Soviet IS, as it was born from KV-13 project of "medium tank with heavy armor" where necessity of some sacrifices was obvious from the start due to ambitious design goals.

Armies entering the war with doctrines that "tanks don't fight tanks" learned that actually "the best anti-tank weapon is another tank". That, increasing power of anti-tank weapons and discovered mobility issues of German super-heavy designs led to the need to increase (mostly frontal) protection while retaining mobility and keeping mass and dimensions manageable (for bridges, for trailer and railroad transportation). Deletion of a crew member and machine gun mount simplifies and strengthens the design of glacis and allows to decrease the internal volume (beneficial for making the tank smaller and lighter) or to use the freed space for ammunition storage (as ammunition grew in size - even German mass-produced tanks didn't mount 100+ mm high-velocity guns during the war). Even though extra machine gun protected by armor and operated by a dedicated crew member (even with limited sector of fire) is definitely beneficial for protection against infantry, design constrains dictated that combined arms and group tactics now had to compensate for its absence.

Tankers toyed with the idea to retain a fixed bow MG fired by driver (T-44, Centurion prototypes), but it was ineffective and quickly abandoned. Designers also pursued the goal to let the machine guns mounted on top of the turret to be aimed and fired from inside the tank (that makes them even better than bow gun - with wide arc of fire and high elevation needed for urban or mountain combat), but only with recent advances in electronics did remote-controlled weapon stations truly mature.