It is mainly to do with structure. If you think of a standard Y class seat the back is very thin (to maximise passenger space) and is designed to fold forward in an accident to reduce head impacts. All of this wouldn't give a fixed anchor point for a car style seatbelt. (You could potentially make the seat back rigid but it then moves the stresses onto the floor which you have to make stronger etc etc).

However, biz class seats nowadays tend to have a rigid shell structure behind them which does provide something to secure the shoulder strap to. Combine that with sometimes limited space and the funky design but very rigid IFE screens or other seat back shell and you start needing to stop the human in a very short distance, thus 3 point seatbelts, airbags and other innovations from cars.

I recently flew on a carrier that had introduced a new biz class seat, with car style seatbelts ...only snag was there wasn't any information on how to "brace" for that type of seat and belt combination. (For those interested it's back straight and head down onto your chin with hands on the lap.)

See pages 7 onwards in this very handy Transport Canada document for some pictures based on the latest FAA research. Transport Canada