"Multiple ledger support will also allow us to use different ledgers depending on the requirements of large corporate clients or industry verticals of our nanopayment settlement platform. This is where our long-term partnership with IOTA comes in. While we didn’t pick IOTA as the default ledger for our web content payment product, we will continue to evaluate IOTA for upcoming products, for example for white-label solutions or in the Internet of things sector."
As far as I know, the main reason is that IOTA is currently under heavy development and it can be that the network doesn't work. For example, when the IOTA developers asked every node provider to take it offline to install an update.
In summary, Satoshi Pay would have used IOTA if it would been production ready.