Why can't you buy or drive diesel cars in Greece?
There is legislation which makes the use of diesel-powered private vehicles illegal in the cities of Athens and Salonica. This legislation has been in force since many decades and is aimed towards reducing the damage caused to ancient monuments by the sooty (particulate rich) emmisions of diesel vehicles.
Without doubt, modern diesels are much cleaner running than their predecessors, but the legislation remains in force today.
The only diesel powereed vehicles allowed to circulate in the major city centres are ones used for "commercial" purposes i.e. Taxis & vans