locos are heavy and need loads of Tractive effort to haul heavy load.
while MU-ing D-locos is plausible owing to their high tractive effort, the same with WAP 3 wouldn't have been possible owing to it's low tractive effort. It was already struggling anyways.
Also, having a loco that needs a supporter on even mild gradients isn't something railways looks forward to, it puts an additional unnecessary demand for locos increasing operational costs. hence theyw ere replaced by far, far more superior WAP 4