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Anomalous oceanic and atmospheric conditions in the tropical Pacific were similar in the early months of 2014 and 2015, both apparently favoring onset of El Nino. Despite high expectations, the 2014 event did not truly take hold, but the 2015 event developed into a very strong El Nino. Here we argue that mean state differences played important roles in promoting El Nino in 2015 and not favoring growth in 2014. We show that the mean state difference in the Pacific between 2015 and 2014 was similar to the shift from cold phase to warm phase of the Interdecadal Pacific Oscillation (IPO). We demonstrate that this mean state change affected El Nino development between the two years using a forecast sensitivity experiment. The cold IPO-like mean state during 2014 acted to reduce the coupled feedbacks, while the warm IPO-like mean state of 2015 acted to enhance the coupled feedbacks.