For one, there was no Warsaw Pact in 1948, when the Tito-Stalin split occurred, and this was still in the early post-War period where many things (borders, alliances) were hazy.

Also, the split was mostly due to disagreements which were discovered over the course of the split. Rudimentary plans of a communist alliance made during the Second World War were interpreted differently and these differing conceptions were only really hammered in post-War. Tito, for instance, intended the alliance to be less unipolar, and supported the expansion of Yugoslavia and Bulgaria far beyond how Stalin had envisioned (specifically, Tito desired Yugoslavia and Bulgaria to absorb Greece, and for Yugoslavia alone to absorb Albania and parts of Italy).

Finally, the most important reason was that, in WWII, the Soviets did not have a strong presence in Yugoslavia, and Yugoslav independence was achieved above all by the actions of the partisans, and not the approaching Soviets, as it had been in other Eastern Bloc countries. So, when Yugoslavia did split, the Soviets had only spies in the country, but few boots on the ground troops vs. Czechoslovakia and Hungary.