Almost all stations have a switcher that isn't sufficient to have every feed coming into the building available on it. A small market may only have a handful of sources, but probably has a small switcher. A large market station may have a hundred sources but a switcher that only supports 50. Most stations I've seen rely on having a block of switcher inputs (I'd say on average 10) designated for remote sources (which can be satellite feeds, microwave, bonded cellular, and in some cases infrequently used hookups within the building itself) that are fed from the facility's router, and usually these run through framesyncs to ensure they're synchronized with the switcher. Since KGO was in the middle of a show, they would already have had the remote shots they were using routed in.
It's possible that they already had CNN Newsource routed to the switcher for something else that was being fed on it (Newsource fed a murder trial verdict from Southern California less than an hour prior to the crash, at the time of the crash, Newsource was offering two feeds from a House Judiciary Committee hearing regarding the Mueller report). It's possible NewsOne was feeding something else and couldn't feed WABC immediately, or in a rush to get on-air, WABC wasn't feeding NewsOne. My guess is a producer, either in the booth or in the newsroom, saw the alert come down from CNN first, and at that point their priority was to get Newsource on the air because they knew that this was being covered on it.