I’d point more to it relying on MDB2 for all the database interactions, whereas MDB2 doesn’t work in recent versions of PHP which broke some things of reverse-compatibility (supposedly in the sake of better performance?), and that library hasn’t been updated for beyond a decade: https://pear.php.net/package/MDB2
It’s that it may entail reworking anything DB-related to move off of MDB2, which could be the whole project, or else fixing MDB2 to run in ‘modern’ PHP. I assume everyone would rather just write something entirely new, but pretty much any of the efforts miss the point, such as not doing: easy moddability via plugins, being able to run on constrained hosting environments, being able to just extract an archive and use a web installer versus expecting everyone to be a Linux neckbeard to use build tools, etc.