Maybe because Apremilast is administered twice daily for 16 weeks, while Prurisol is administered twice daily for 3 weeks. A 16 week course of Apremilast will be in the area of $8,000, cheaper than Humira and Stelara (~$13K) against both of which it is inferior, but still expensive. Maybe even because in the previous Phase 3 (to which you allude) it only achieved 33% PASI-75, the benchmark for efficacy, which is a dramatic drop from the 41% PASI-75 in previous trials.
The above all seem like good reasons to take on Celgene who only expects to capture 10% of the psoriasis market.
I would suggest some posters here read about Celgene's Aprilimast and its success in phase 3. It will soon capture the psoriasis market and will be a global success. Why would Leo want to take on Celgene and it's product when he can rapidly progress Brilacidin with the limited funds they have on hand.