It is simply because the ES series is the latest Motif series so the price on older stock always drops when it is superseded. Of course the ES models have newer features such as expanded waveform memory and more sample RAM but they share the same basic sound engine.

I own a Motif 6 that I bought when its original owner decided to move up to an ES Motif. I decided the original Motif was the most cost effective solution for me. I guess you need to decide whether the extra features justify what they will cost you.