I would have thought the economics of running a 300 vs 400 would have been similar (scaled for the extra 30/40 pax) so i would share your curiousity TB.
A quick google search showed that the 300 outsold the 400 by 2.5 to 1. So maybe it was all about not finding available aircraft. then when the flood of used 800's appeared on the scene, it was the ideal time to get in on the action. Before Jet2's first 800 in 2011, they had use of B733's and B752's, so they knew about selling more seats than the 300, but on slightly weaker routes than the B752 could handle.