Why is the TV show "Tehran" filmed in Athens?

According to this article:

Not only are both cities very ancient, but Herzberg says they have also undergone “massive urban restructuring in the 20th century”. This has seen the introduction of modernist architecture and urban planning into old cities. As a result, Tehran and Athens have a similar mixture of what Herzberg calls “glum modern, Neoclassical and European-style buildings”, each of which are low rise and have either Greek or Persian architecture.

Herzberg also notes they have “narrow residential streets and alleys” that are “combined with wide boulevards and squares”, while even their “lower-income neighbourhoods are not dissimilar, too”. As a result, they did not have to create any new studio sets or fake facades for filming.

Bejach says these similarities were not only restricted to the city centres. “Athens is a city surrounded by mountains, exactly like Tehran. Athens is also very green with gardens resembling Tehran.”

As you point out, filming Tehran, was not possible given the political tensions between Israel and Iran - for example, recently, by Iranian accounts, Israel assassinated their top nuclear physicist.

The production team had to find a location that was mediterranean, surrounded by mountains, with narrow alleys and gardens with a profusion of jasmine and bougainvillea' and the production team felt that Athens best fit this bill.

On a personal note, I can vouch for the 'narrow alleys' of Athens since I've been to the city and seen them for myself.