The major reason for the absence of Yugoslavia from the Warsaw Pact of 1955 was that the Soviet Union and Yugoslavia had already essentially split in 1948.

Unlike other eastern European communist regimes, Tito and his party were much more independent from Soviet authority. They had fought a civil war as well as the occupiers and in many respects liberated themselves in 1945.

Essentially, Tito wouldn't toe the Soviet line and refused to simply take orders from Moscow. Tito provided material support for the Greek communists despite Stalin wishing to stay out of that conflict. Tito also had his own aspirations about the role of Yugoslavia in the Balkans.

Yugoslavia was expelled from Cominform (the international communist party organisation) in 1948, after a series of disputes, meetings and exchange of letters. The idea of 'Titoism' emerged in the Eastern Bloc - that states could take their own national 'road to socialism' that differed from the plans set out by the Soviets, and purges of supposed Titoists occurred.

Yugoslavia was an integral and founding member of the Non-Aligned Movement as a result of this split, and never really was a part of the Eastern Bloc in the same way that other states were.