What is the reason for this difference? The desired output or the need for different processing for logistical reasons? (I know nothing about ECN2)
ECN2 film is designed to be printed to different media - projection stock, designed for ECP development.
The film's three colour emulsions are contrast matched to the colour emulsions in that projection material.
C41 films are designed to be printed to paper that is designed for RA4 development.
Their three colour emulsions are contrast matched to the colour emulsions in that material.
Optimizing something for a projection medium is always going to lead to major differences from something optimized for a reflective medium.
In the real world now, both types of films are also set up for digital intermediate steps, but the optimizations are still very different.
And you get crossover when the contrast behavior of the three colour emulsions isn't well matched to each other, when the characteristics of the presentation medium are taken into account.