Simple and reliable technology the could be fitted to signals and Locomtives to stop a train that was doing a SPAD (Signal Past at Danger) had been known since the late 1800's, but it was not made manditory even in England till just after the terrible crash of three passenger trains at Harrow and Wealdston that killed 112 people and injured another 300 in 1952.
There were lots of inventors patenting things in the late 19th century, but almost none were successful.
The first successful warning system was the French 'crocodile' which dated from around 1883. However, at this time, it was not failsafe, only provided a warning at the distant, and did not apply the brakes.
The trainstop was the first actual mechanism that would stop a train SPADing. The first successful example was on the Boston Elevated and dated from just after 1900. Mechanical train stops were never popular outside metro systems. Big railways always rejected them - typically this was due to the fear of damage to the train stop at high speeds.
The GWR was the first to deploy a failsafe mainline system on a wide scale - starting around 1908. But this was still only applied at distant signals. It would apply the brakes if the train passed a distant signal that was on, but not if it passed a stop signal. The NER had another system - the Reliostop - which would apply the brakes on passing a signal, but this project was closed down on Grouping.
After WWI the US ICC mandated that major railways each had to fit a division with an automatic control system. All did so, and a number of different systems were installed. With falling passenger numbers in the '30s, almost all of these systems were either ripped out, or converted to cab signal systems (i.e. no control over the brakes). The PRR was the only system that really adopted and extended its system.
Little else was done until post WWII. You are correct, that Harrow was the immediate impetus for the adoption of the UK AWS system. But, like the GWR system, this provided a warning when passing a signal at caution (and applied the brakes if the warning was not acknowledged). It did not apply the brakes if passing a signal at danger. Even after 'adopting' the technology, the British weren't particularly interested in deploying the system. It was only slowly extended, and only as part of a package of resignalling with automatic signals. There was certainly no general installation program.
Post WWII, European railways did introduce automatic control systems - Indusi is the obvious example. These were not failsafe and regarded askance by the British.
Considering the state of the art in the late 50s when the signalling for the standard gauge was developed, I would say:
1) the technology certainly existed, but it was clearly not thought to be worthwhile to install (certainly not the UK or US)
2) it wasn't particularly cheap (this was why the various railways always hastened slowly)
3) there was no obvious need to install it - there were few high speed accidents caused by SPADing.