Practically it’s because Rise of Iron WAS filler. In-universe it’s because we destroyed the SIVA replication complex under the Cosmodrome, ending the Devil’s production. This triggered Aksis calling all Fallen to his banner with any SIVA samples as payment for their ascension. We intercepted this transmission, stormed their base and destroyed their plans for an invasion force while crippling the Splicers. The Cosmodrome was then sealed off under Quarantine. Bungie however disregarded Rise of Iron when the Cosmodrome returned, with some of the destruction caused undone for no apparent reason. Any remaining SIVA would likely only be found in the Exodus ships. Neomuna was founded on it and it was eventually upgraded into the nanotech we see the Cloudstriders use as well as Quicksilver Storm. The Exodus Black’a store is classified so Failsafe cannot locate it. It’s either been unclaimed, scavenged by the Fallen or found by the Vex. There’s also Outbreak Perfected, a small SIVA generator that Eramis attempted to steal in order to win over any remaining Devil Splicers. Her Lieutenant failed in that mission and we resecured it.