Because they want only ONE format to survive. It is cheaper to produce, stock, track, etc, a single format. They picked their horse.
Some liked that the replication plants had to be new. This would slow counterfeiting.
Some like the larger capacity. This would not only allow more extras, it might allow a second or third movie on the disc. Users could then pay up, get a code and unlock those movies. Max demonstrated capacity (in the lab) is 200 GB on 6 (some say 8) layers.
Some like the regional coding ability, and the watermarking. Some are counting on the BD+ security system to arrive and save the day, DRM wise.
Maybe some saw the computer industry going whole hog Blu-ray and saw the writing on the wall.
Maybe some got a deal from Sony they couldn't refuse.