Jason Blumhouse has stated he isn't an idea guy but he can spot a good idea. James Wan is a idea guy but doesn't always do well on execution. Combining them together with Universal also getting a share means Universal gets a great producer who makes great movies at lower budgets, an idea guy who will use the producer to execute his ideas, and a pipeline for new films in their lineup. Its a win win.
Also outside the Conjuring, he really didn't make anything that WB didn't already own looking at the released stuff everything else was already established IPs under WB.