For starters the different versions could be produced for the movie company by different companies in different factories.
There's also the issue of licensing costs in relation to the protection systems.
The company that markets the protection usually gets a kickback from each disc produced.
They may well consider the likelihood of someone that has the Blu-ray pirating the DVD as less of a risk and are trying to save money on licensing fees.
I've seen this in the past on quite a few other releases so it's not as rare an occurrence as people might imagine.