Well, my guess is - When an aircraft crashes "conventionally" (ie, it's flying forward and hasn't broken up in the air) its high forward velocity very rapidly reduces to zero.
Therefore the forward inertia of passengers seated facing the front causes their bodies to be flung forward against their seat belt and into the back of the seat in front of them with resulting serious, if not mortal, injuries.
Whereas if they're seated facing the rear the dissipation of their forward inertial energy will be spread against their seat back, which "should" reduce injury - similar in concept to "baby capsules" used in cars.
This of course does not take into account other factors like the direction of impact, structural integrity of the seat and the airframe, fire and explosion, etc.
Besides which most pax like to face their direction of travel.
I must say that I felt better looking at the blurred scenery whizzing past the window of the Eurostar train whilst travelling at 300kph when sitting facing forward then when facing back.
Many early post WWII airliner designs proposed rearward facing seats, but like QWERTY keyboards some things are just hard to change people's mind about.