Jean and Ororo bonded pretty fast even before Jean became Phoenix. This is just a few issues after the debut of the ANAD X-Men, and after Jean had technically left the team, and Ororo still mentions not only the hours they've spent talking, but
confiding in each other as well.
I see it as 2 highly intuitive, empathic, elemental women bypassing the barriers most people erect around themselves and quickly finding the core being of the other that resonants with themself.
I don't think it's fair to say it's just because they were the only two female members because when Lorna first joined, rven though she and Jean became friends, it didn't get to that "soul sister" level that Ororo and Jean formed, and so quickly at that...also, as the only two female members, they could have easily became enemies/rivals if Claremont chose to play into that stereotype(say, as a companion to the Cyclops/Wolverine rivalry that was already bring played up.)