The same reason Yoda's voice is in in the film (though I didn't hear it) it's because, like Yoda, obi-wan would have detected a new strong connection with the force.
But Jinn I don't think ever mastered being a force spirit...he could only project his voice not something you could see. If I recall he could only 'appear' on one certain planet. The three I mentioned I assume must have refined the teaching he discovered. But yeah I guess he also would detect...
I'd assume Anakin, Yoda and Obi-Wan - the three Jedi who are able to appear as Force spirits and are able to detect changes within the Cosmic Force. They would have been aware of Rey's 'awakening'.