The development of wider 4-sprocket clusters exagerated axle overhang and thus axle failures. The 1980's introduction of 6-speed and 7-speed freewheels only made the problem worse. The obvious solution is a larger axle, but the total axle plus bearing diameter is constrained by the size of freewheel threads, and bearing cone pitting was already a problem, suggesting smaller bearings would be a poor tradeoff.
That said, broken axle problems can be solved with freewheels. 1970's freewheel hubs such as Phil Wood and Bullseye have larger axles and markedly fewer axle failures. However, hubs with oversize axles were markedly more expensive, Fitting oversize axles to cup-and-cone hubs requi Alternatively, a shoulder plus spacers would reduce loads in the axle's threaded section and thus breakage. My (possibly faulty) recollection is shouldered solid axles were used by at least one maker, but they were not widely adopted. and makers of low-price hubs chose not to innovate on either path. Curiously, many modern freehubs use a layout like the Bayliss-Wiley, which has as bad or worse overhang as on failure-prone freewheel hubs, but better designs fix the broken axle problem.