As I recall, much of the art in McQuarrie's ROTJ portfolio was done "after the fact," being made especially for the portfolio and not for the film itself. Which is why many of the ships and creatures in the paintings look suspiciously like their final film counterparts.

If ole Ralph didn't like the direction the film was heading, maybe that's why he stopped doing art for it until it was a finished product. At which point he would bear no responsibility for the quality of the movie itself--instead of being the proud concept artist behind the look of merchandise-driven teddy bears.

Apparently he was happy to work on the film early on though--witness all the concept art he did for the original draft's throne-room sequence on Coruscant. Must be that Lucas and his terrible ideas drove him off (haven't we heard this story before??).