This is down to ever-tightening CO2 emissions regulations, which means "the current 2.5-litre four-cylinder turbo boxer engine simply can not continue to be used in the future", at least in Europe.
Although no decision has been made regarding the new WRX STI's drivetrain setup, the company is said to be investigating all possibilities, including a hybrid.
Above: Subaru Viziv Performance Concept.
At the recent Tokyo motor show, Subaru unveiled the Viziv Performance Concept, which was widely seen as a pointer to the next-generation WRX. With the company's past form, the production WRX will likely sport a significantly toned-down exterior design.
The company did not provide any drivetrain details for the Viziv Performance Concept other than it has a boxer engine and all-wheel drive, providing us few clues about the new WRX's drivetrain.