For Turkey, seeing PKK and YPG gain territory that increase chances of creating an independent Kurdistan is a nightmare, and if a Kurdistan is established in Syria and Iraq (Given how unstable those countries are) this will increase the chances of eastern turkey (which is predominantly Kurdish and has seen lots of insurgency by the Kurds and harsh reactions by the Turkish government) seceding to this newly formed state.

The AKP is formed by islamist Turkish nationalists and Turkey has a habit history of committing genocide (Greeks, Assyrians, Armenians). Turks and Kurds had some form of 'Peace' until Erdogan bombed Kurdish bases in Iraq, turkey wants to get rid of the Kurds and destroy any chance of a Kurdistan, they are hindering the closure of this war because the results most likely will not be in their favor.

Even though they do not support ISIS, turkey has turned a blind eye to a group that is a threat to everyone including them because they fight their other enemy, Turkey wants them to fight each other to extinction but that is not going to happen, and unfortunately for turkey it looks like the Kurds are winning.

ISIS has it's influence in Africa, South Asia, Central Asia and South East Asia, not to mention their influence in Europe and America, they want to establish a world wide caliphate and to achieve that goal they will continue to try and make as many terrorist as they can out of muslims.

The Kurdish conflict is contained within Mesopotamia, there is an end objective to it. and their fight is only with turkey since in Syria and Iraq they basically run autonomous regions.

So the rest of the world tries to arm the Kurdish factions because they fight ISIS and also because Kurds align more with western values, why strain your country when there are already ground forces fighting your enemy and also agrees with your views, Turkey does not like this because of obvious reasons. If it really comes down to saving the world at the cost of turkey loosing some territory, probably rest of the countries are going to do the obvious and continue to support the Kurds.

Turkey can support only the rebels in this war because having Assad in Syria who is a friend of Russia which turkey has blatantly defied is not going to turn out well, ISIS wants to bomb them and the Kurds want independence.

just geopolitics...