Jack Welch was a
massive GE Capital fan - he praised it to the skies in his books and it really came into its own under him. He said (and I'm paraphrasing) that as a chemical engineer by training, he was blown way that you didn't have to design, manufacture, "bend metal" with GE Capital - just turn GE's strong balance sheet into more money.
Maybe it's possible that that type of culture and system made Immelt the best candidate to take over? Or influenced his own decision-making?