Three folds. First, they only recently introduced this sensor, which is great, but no better than the competition. Second, they are going against a true giant, Arri. Given a budget, any DP would take an Arri over anything else. Third, their cameras traditionally aren't that reliable. A feature film costs about $150 million to make. One hour downtime is crazy expensive.
My perception of RED is that the hype doesn't quite match the product, and I actually don't think they sell a lot of cameras. The expensive ones are sold to rental houses, not individuals, and they all have Arri and Venice and Panavision, and the new Raptor-killing Sony Burano is now shipping, so I don't think RED brings much to the table at that end. On the content creator side (sub $10k) they are going with essentially 1 model, with dubious reliability against Sony's 4 models, Canon's four models, Blackmagic, Panny, Z Cam, Kinefinity, soon Fuji, and that's before you count the hybrids that are getting there on image quality, internal RAW, 4k/120, etc.