Some times the boos are fair (like Greece giving 12 to Cyprus in 2010, no one in Greece would've voted for that song if it were from say Estonia or something, the song was in English and not popular in Greece and the band was unknown), but this time the singer is from Greek, the lyrics are partially Greek, the song has Greek sounds, filmed in Greece.
Greece and Cyprus have the same music industry, so a lot of the singers representing one country are already famous in the other. For example Tamta, Demy, Eleni Foureira are all popular in both countries for many years before they did Eurovision. In recent years Cyprus' Eurovision singers have always been from Greece and not from Cyprus, at least for the last four years. In the past Greece also sent several artists from Cyprus (Anna Vissi in 1980 and 2006, Loukas Yiorkas in 2011, Eleftheria Eleftheriou in 2012, Maria-Elena Kyriakou in 2015, and probably some others I am forgetting).
And of course the history/politics play a role as most people from Cyprus are Greek-Cypriots. It's a complicated history which I won't go into detail about.