The 60 knot callout is airspeed, because airspeed is what matters to the reverser and engine. The primary reason you don't want more than reverse idle at 60 knots is to prevent re-ingestion. Re-ingestion can cause a surge or FOD damage - again it's airspeed that is the primary determinate of re-ingestion (there are some potential FOD scenarios where ground speed also comes into play, but those a relatively uncommon).
Using ground speed would effectively mean different procedures for head wind and tail wind landings unless you want to use both (e.g. 'when airspeed or groundspeed is less than 60 knots').