Fundamentally I don't know what the cause is, because there is no full safety report yet, but I doubt that it was a driver error alone, which is what automatic braking systems at signals are actually targeting.

There are situations where the dispatcher needs to instruct the driver to ignore the road signals and hence also ignore the automatic enforcement of the signals. It shouldn't happen to often, but the real world is always more messy.

Some were installed. But what wasn’t there was the old mechanical ones that cannot be overridden, like ATS that blows the air if you pass a red signal.

https://en.wikipedia.org/wiki/Automatic_train_stop

Any failsafe that can be bypassed manually is just a deferral to humans and eventually will result in disaster.