The technology for the Volt still exists. Rumors are that an SUV-version of the Volt is coming (just like the electric-SUV version of the Mustang is allegedly in the works).

Sedan marketplace is shrinking, SUVs are growing. GM doesn't want to waste its hybrid technology, but also doesn't want to bet on the Sedan market anymore.

EDIT: Note that a 20-mile range PHEV (the typical one) is insufficient. 50-mile range PHEV seem to be the sweet spot for me.

The Volt was cancelled because the Cruze was cancelled so it doesn't say much about the PHEV market.