Two cents worth here......
KTM bought out Berg and saved it btw for the motor design. The original KTM motors were good, but were more old school and frankly a bit of turd.
Husaberg has been doing a lot of R&D for a lot of years on motors and chassis........have you ever wondered how much of the current KTM motors are, or could have come from the Swedish R&D?
The 70* motor solves one of the biggest problems and or characteristics in handling, the Term (math formula) that relates the rotating mass of the flywheel and its gyroscopic effect, and its subsequent distance from the center of the mass of the bike. Also, take into account the Term of the reciprocating parts relative to the axis of roll, pitch, and yaw. Combine the lessening of these forces, and you get the quick handling of the 70* design, yet it is still stable.
So even if you make the motor a few pounds lighter, it does not negate the Term of the distance of the rotating mass from the center of the bike.
Another case in point, and I'll probably get flamed for this one, take the gas charged Closed Cartridge forks. They were designed for SX and MX, and do not work as well as the open cartridge forks OFF ROAD. And to back up my point, you will notice that all the off road KTM's still have the OC forks. The change to the CC forks on Husaberg's were mainly a marketing decision IMHO, which, I'm betting is the decision for the dropping of the 70* motor design. Anyone, and I mean anyone, who has ridden a properly set up 70* motored bike will have to admit that they work better in their intended environment than the typical design.
And there is an economical side, the new KTM motor cases are die cast, where as in the past they were sand cast, so I would imagine that die cast cuts production costs, and perhaps KTM did not want to invest in the dies for the 70* motor.
And lets not forget the man who headed up the 70 project left HBG in this last year if I am not mistaken.