The failure to arm the spoilers was the result of continuing an approach that exceeded safe criteria. The main reason we have stabilized approach criteria is that MANY studies have shown that unstable approaches place a high level of workload upon the crew and this is fertile the environment for skipped checklists and pilot error. It has to do with human factors.
As far as the spoilers are concerned, the fatal error was in not confirming them deployed once they touched down. Even when armed, spoilers do not always extend correctly. It is not enough just to arm them on the checklist. Same goes for autobrakes.
Studies have also shown that pilots routinely continue into active thunderstorms. Get-there-itis. Plan Continuation Bias. Call it what you will. The problem lies in the human vulnerability to poor judgment.