A great question. Ocean temperatures across the tropical Pacific Ocean naturally tend to have their greatest difference (from west to east) during the northern hemisphere winter. The cold tongue which extends across the central/eastern Tropical Pacific is also climatologically at its strongest and extends farthest west at this time.

This helps to maximize the impact from a warming (or cooling) of waters in the central/eastern Pacific associated with El Nino (or La Nina). Which then helps to make the Nino3.4 index the largest at the same time.

It is this gradient in SST's which helps to drive the atmospheric response in El Nino. Maximize the gradient, maximize the impact.