Because life is cruel.... Also I guess it would seem from the movie that Helen was becoming unstable, causing the amount of time between changes to be harder to predict, as we can tell by when Richard Miscalculated trying to em-Bare-ASS her at work by destroying all the clothes he had on at work thinking he would change into her, and then also when Helen unexpectedly changed into Richard At Sarah's Party leaving him dolled up in an orange dress.