Most if not all such managers (I always look for explanations in these cases that are not ascribed to an abstract business entity, but to individual people) have no stomach for porting. A typical manager's tenure over an organization that might have the scope to perform the port is shorter than the porting project itself. No manager wants to have such a large, expensive, risky project on their accomplishments list as "started, but not finished", especially if a successor gets to claim all the credit later if it does finish successfully: that successor then becomes direct competition on the corporate ladder climb.

Also, I've yet to see such businesses opt for the safest porting approach. Build a transaction replication system, then a component of the ported system. Inbound transactions are replicated and dispatched to the legacy and the component of the ported system simultaneously. Build scaffolding to monitor key transaction metrics. Compare output and performance results. Let that "bake" for a year or two. Rinse and repeat with the next component (many such ported components can be developed in parallel, etc.), and so on, until the ported system reaches feature parity with the legacy system, then let the completed version "burn in" for 2-3 of the longest periodic process lifecycles. For example, if the the longest periodic process the application supports is once every 7 years, then you burn in for up to 21 years, comparing during the entire duration.

By the time you "throw the switch" to the ported system, you have a very high certainty that it will work just as well if not better than the legacy system. Extremely low-risk, but extremely expensive, and considered extremely impractical. And with how business-critical some of these systems are, pretty much any risk mitigation short of this will get nixed by senior management.

This doesn't even begin to get into other factors, like changing regulatory and business environment as you are porting (so you're porting and modifying for new requirements simultaneously), such a project can easily be targeted for the cutting block for just the cost savings to make someone in Finance look good, internal political issues, developer churn over such a long period, developers on the legacy system withholding key implementation or even business requirements information, that information simply not being available, binary-only parts of legacy, etc.