There exist two equilibrium states for the tropical Pacific--one is zonally symmetric (or nearly so) with the warm-pool extending all the way to the eastern Pacific, and the other is strongly zonally asymmetric with the warm-pool confined to the western half of the tropical Pacific. ENSO results from the fact that under the current radiative heating, both states are unstable, resulting in the apparent "wandering" behavior in between these two states as seen in the observations. (In other words, the coupled tropical Pacific is an bi-unstable system under strong radiative heating). This theory concerning the origin of El Nino was first proposed by Sun (1997) using a box model for the coupled tropical Pacifi ocean-atmosphere system.
A suprising apsect of the Sun model of El Nino is that it simulates the ENSO asymmetry, an observed aspect of this phenomenon and has been difficut for the previous theories, such as the delayed oscillator and the recharge and discharge oscillator to account for. Related to its success in simulating ENSO asymmetry, the ENSO in the model is also featured with the existence of extreme El Nino events and more generally the existence of a rich spectrum for the magnitudes of El Nino events.
The Sun model also provides a pedgogic case to show that the climatology is a not an equilbrium state of the system, rather, a strong function of the amplitudes of ENSO
But most importantly, this diabatic and nonlinear view has shedded light onto the central role of the intensity of heating. It has provided a mathematical theory to explain why the coupled system has to "relax" once for a while in the process of moving heat downward (and thereby poleward). In short, the answer from this theory to this question is that the "work load" is too heavy. The theory also shows that both a state characterized by a permanent El Nino and a state that is characterized by a permanent La Nina are both solutions of the system, providing an opportunity to account for such behaviors that have been suggested in the past climates.
The centrol role of heating is consistent with findings from a series of observational and modeling studies.