I don’t know if it’s _the_ reason but a possible reason is a protection of US citizens rights.
(If it’s indeed true that the CIA only looks at what’s going on ex-US)
But I doubt the two would play nice with each other if the flow was only 1-way.
The FBI's goal is to arrest people, charge them, and send them to jail. Everything they do will be presented to the public, at trial, and examined by the opposing side and presented to a jury. There's not a lot of room for nuance here. There's also not a lot of room for secrecy. The accused has the right to confront their accuser.
The CIA's goal is to "protect the nation" and almost by definition deals entirely in secrets and ambiguity. "Sources and methods" are some of the most closely held and valuable secrets. "A person we kinda trust, sometimes, accused someone of possibly planning to maybe do something nefarious." -- not going to hold up well in court, even if it's totally reasonable to now put that person on some kind of watch list (and yes, at this point, all of us are on some sort of watch list -- see the disclaimer above; we're now at 30,000 days of the condor and counting)
So -- intelligence sharing between the CIA and the FBI sounds great in theory; in practice it may be tricky.
The CIA, NSA, etc. aren't allowed to track domestic stuff; they're codebreaking and/or foreign intelligence.
But they sure as hell can share their tools with the FBI, who are authorized to do domestic surveillance.
Love the reference, BTW :-)