Of all the nations who lost in WWI and were subject to more or less harsh peace treaties, only the Ottoman Empire managed to reject theirs, and later signed the Treaty of Lausanne which provided them with much more favorable conditons.
Why could they do that? Why couldn't e. g. Germany reject the Treaty of Versailles and later come up with a more favorable treaty?
Since nations are sovereign, they can in practice reject any treaty at any time. It's up to the other parties to enforce it. Germany also latter violated the treaty of Versailles.
In Turkey's case, they rejected Sevres and successfully fought off the Allies (see Turkish War of Independence), and afterwards Turkey and Allies signed another treaty.
The Ottoman Empire did not reject the treaty. The Republic of Turkey did. Ataturk had to fight the remnant Ottoman monarchy along with the European invaders. The Entente went along with it because it was tired of war after 4-5 years and was divided on how to tackle Ataturk (the US was never at war with the Ottomans to begin with and the only one who cared about the area was Wilson, Canada refused to send troops to Turkey, etc).