Because Apremilast does not CURE psoriasis but Prurisol has the potential to CURE it.
"During a phase 3 trial back in March, a third of the patients given apremilast showed a 75% reduction in psoriasis in comparison to a placebo. However, that was considerably less than the 41% that had shown the same reduction of psoriasis during an earlier phase 2 trial -- which indicated that the drug might not be an attractive alternative to market dominating biologics from AbbVie , Johnson & Johnson , and Amgen."