She noted that the idea of the "perception filter" gets used a lot in Doctor Who and wondered why, in that case, the Slitheen went to all the trouble of wearing body suits.

Production-wise, I suppose it hadn't been thought of yet? Can't remember whether it had already been mentioned by the time the Slitheen showed up. Is there precedent in classic Who? (I don't have enough of that under my belt to know; the selection on Netflix is small.)

In-universe, I suppose Raxacoricofallapatorius doesn't have that technology for some reason?