Why doesn't the Convair 580 have check valves to prevent inadvertent fuel transfer?
A check valve is another failure point in the fuel system (they can stick) so you have to do a risk analysis to decide if the benefit of adding them outweighs the risk of the added failure mode introduced, with sufficient weight to justify mandating them by AD.  With no AD, it's up to KFC to decide if it wants to do anything on its own.

Assuming this was a one-off event (the report only mentions one previous less serious  but similar event) that is more or less attributable to bad piloting, the balance of risk, when added to the burden of forcing an AD on operators, would have been determined to not fall within the threshold needed to mandate such a change (I've participated in these kinds of projects over the years).

If another crash or two happens later with more or less the same circumstances, it's more likely to trigger an AD.  But these airplanes have been flying a very long time like this.

Yes there is a tendency to think the need to do something is obvious, but if ADs were issued for every potential design deficiency to cover every eventuality, there would be eleventy jillion ADs out there.  That being said, there was probably an internal debate with discussions between Transport Canada and KFC, with pro and con factions arguing it out, and TC was probably convinced that mandating check valves would not provide sufficient risk mitigation to justify the burden of issuing an AD (The FAA could do something on their own, but usually they will work with TC on this and go along with TC's resolution).