Because the PIC is xtal locked giving very good accuracy and stability for the 60Hz, AND the PIC can drive push-pull PWM which will make an efficient sinewave inverter. And the PIC uses almost no analog parts besides the xtal and its 2 caps.
It´s interesting, but the Microchip PICREF-1 UPS also has a feedback circuit. It´s not so complicated to design a sine-generator (the digital method is recommended, because it is more stable) and the easiest way is to use a DDS, for example AD 9833 from Analog (https://www.electro-tech-online.com/custompdfs/2009/12/AD9833.pdf). One challenge with this cirucit is its uSOIC-size; only 5 x 3 mm with its 10 pins.
The "real" problem is to implement the feedback-function, where there is a risk that the device will start to oscillate. So, as I mentioned in my earlier mail, the challenge is to design the output filter AND compensate it, either in a analog way (Error Amplifier) or digital (Formula, see PICREF-1).