PCI compliance might be one good reason.
If you're referring to PCI requirements section 9 - "Cardholder data must
never be stored on a server connected to the Internet" - that doesn't
preclude you from running HTTP servers on a database server. Would anyone
suggest that running QZDASOINIT jobs and ODBC/JDBC interfaces on an IBM i
server are more secure?
No, PCI 9 means storing cardholder data on a network segment which is
different and separate from your Internet IP address. And to make the
interfaces even more secure, you might use an Apache HTTP reverse proxy on
a Linux server, forward traffic to your IBM i HTTP server - which would be
unquestionably PCI compliant.