This was a very rare event: in the majority of cases, they're able to get volunteers to take a later flight. In almost every single one of the cases where they aren't, the people involuntarily bumped leave without incident. I doubt anyone on the flight crew foresaw their chain of actions ending in violence, right up until it happened. And I think that lack of foresight is entirely reasonable and understandable.
United (as a company) did a poor job of handling the aftermath, though.
The cost of involuntarily bumping 4 passengers ($800+ x 4) assuming no further incident, rapidly approached any benefit from not cancelling (or delaying) the morning flight. Total revenue for the regional flight would likely be less than $10,000.
Seriously, this wasn't hard.
Stupidly, The cost of the likely lawsuits (even from lesser incidents, that didn't involve beaten passengers, just extremely annoyed passengers) often negates any cost saving for the airline.