If they had observed the modern industry-standard stable approach criteria, then they would have gone around at both 1000' and failing that 500'.
I see the failure to go around off an unstable approach as the primary cause, because it is a fundamental mistake, it is the first mistake and it would have arrested all the subsequent mistakes.
After that, it is clear that the PF was flying it as if he were in an Airbus (static levers, a/t on), when he in fact had the a/t off. A poor Airbus pilot has poor speed monitoring, because the standard way to fly a visual is with a/t on, and the a/t does an exceptionally good and reliable job.
Therefore he was not monitoring speed, and the levers were doing "as expected" (ie as per the Airbus - nothing).
But that's all irrelevant - he flew a criminally bad intermediate approach, and neither pilot observed the stable approach criteria. The real answer to why this happened, is why did they not go around at either 1000' or 500'? Training, company culture, negligence, recriminations, SOPs, something else...
Qualification: I have gone between Airbus and Boeing several times in my career.