You're right. The Greek oriented Orthodox have marginalized "bishops," who are not metropolitans and archbishops. This practice has been going on since the 1920's. I recall, during the post 1978 era in the GOAA, when our regional axillary bishops became diocesan bishops of the North & South American dioceses, an old Greek from Greece friend of mine, told me he didn't understand the change, why weren't the American bishops given metropolitan status like the bishops in Greece. He said kiddingly, "The bishops in Greece are considered 'candle attendants,' (candilanaftis)."
I asked a similar question to yours on this forum a few years back, and no one came up with a specific answer as to why the bishops role became so marginalized in Greek practice, and why in Slavic practice, the change did not occur. I once read in "Orthodox Tradition," the American Exarchate's publication of the Greek Old Calendar Synod in Resistance, that in exchange for accepting the Revised Julian Calendar, the bishops in Greece were elevated to the dignity of metropolitan. Comments about that claim on this forum disgreed that the Calendar change had anything to do with this practice. So, the question remains.