That probably is the reason, although I spotted a few ATR’s operating around the Med.

From the Flightradar24 site, is this actually correct about only 60% carrying ADS-B equipment?

Flightradar24.com shows live airplane traffic from different parts around the world. The technique to receive flight information from aircraft is called ADS-B. That means the Flightradar24.com can only show information about aircraft equipped with ADS-B transponders. Today about 60% of the passenger aircraft and only a small amount of military and private aircraft have an ADS-B transponder.