It’s a fairly complex issue, and I’m not an expert, but here’s my general understanding:
The Ottoman Empire had been known as The Sick Man of Europe for centuries. Once a dominant and feared state, by the 1700’s, it had been surpassed by the various European Great Powers. Prior to that, the OE controlled the Balkans and beyond. It almost conquered Vienna at least twice.
But as time passed, and the OE slowly weakened, the Hapsburg empire (Austria, later the Austro-Hungarian Empire) and Russia became rivals for dominance in the Balkans. To some extent, they kept each other in check, preventing either from outright seizure of the Balkans and the Straits. Other Great Powers (most notably Britain) stepped in when necessary to prevent Russian control of the Straits, as this would allow the Russian Navy to exert power in the Mediterranean. An example of this took place around 1850, when Britain and France joined with the OE in the Crimean War.
In addition to its own interests in expanding control in the area, Russia saw itself as the patron of the largely Slavic and Orthodox peoples under Ottoman control in the Balkans. Slowly, some small independant and quasi-independant states emerged in the region in the 1800’s. Austria had influence in some, and Russia in others. Popular pro-Greek sentiments in Britain resulted in British support for Greek independence, but the UK still had an interest in propping up the OE as a buffer to Russia.
Around 1900, several small Balkan wars and crises increased the size and independence of the Balkan states. Austria had been weakened by various events in the second half of the 1800’s and Russia was thought be be the stronger of the two. Still, German backing of Austria and Germany’s rivalries with Russia combined to induce Germany to lend its weight to prop up the OE, even as the UK became friendlier with Russia.
Of course, rivalries in the area triggered WWI in 1914. But even though the OE was on the losing side of that war (aligned with Germany and Austria), Russia also had been defeated. That, together with the Russian Revolution that immediately followed, meant that none of the victors had any desire to support Russian expansion in the area at Versailles (the treaty that ended WWI and generally reshuffled European borders.)
Not all that much changed in the years betwen the world wars. (I’m ignoring some significant internal changes in the OE, now known as Turkey, but these didn’t immediately lead to significant change in the area externally.)
In WWII, Turkey was neutral for nearly the entire war, joining the Allies at the end. Therefore, no diminishment of its territory occured as part of the war.
During the Cold War, the US took over the UK’s historical role as a prop of Turkish independence (against Russian desires to expand). Any Greek desires to expand to the detriment of Turkey in this century have been thwarted by this. (Both are now NATO members, although there have been flareups between them over the years.) In addition, Greece has never been strong enough on its own to militarily defeat the Turks. They’d need any ally to have a chance, and the obvious historical candidate (Russia) was (a) not attractive to most Greeks as an ally; and (ii) any moves by the Greeks in this direction would have been thwarted by the US.