Thats a very large question, I'll try to summarise all the relevant factors.

Economic success. Tito's economic reforms transformed the country from a rural third world nation into a competitor with western powers. By the later period of Tito's reign Yugoslavia was a large exporter of consumer goods to the West. Importantly, this economic success was shared amongst the majority of the population and great investment was made into public services.

National enemies. Yugoslavia was surrounded by and under threat by foreign powers from the beginning of Tito's reign. First the Nazis, then the soviets and the Marshall plan nations. Common enemies kept the regional rivalries within Yugoslavia in check. Tito was also able to play each side of the cold war's fears off against each other. Both sides were too afraid of Yugoslavia joining the other to pressurise Tito.

Anti-Nationalism. Tito had a special hatred for nationalists (meaning regional nationalists: Serbs, Croats, Kosovans etc.) and would arrest and execute these groups at the drop of a hat. This was likely because of the internationalist aspect of communist philosophy that Tito aspired to, attempting to erase national boundaries. Tito would relax this in later years, choosing to allow moderate nationalist groups to exist and eventually allowed traditional ethnic reason a degree of autonomy. Some believe this autonomy is what allowed some regions to fall into debt and leave them vulnerable to the economic crisis that would eventually help break up Yugoslavia.

Tito's cult of personality. Tito was very good at self promotion. The story of his escape from the Nazis and his victory against the fascists became a national myth. This combined with Yugoslavia's diplomatic and economic successes made Tito into a more than human figure in his people's eyes. Whilst Milosovic was only ever a Serbian, Tito became something that all Yugoslavians had in common, and could rally behind together.

The converse to this are the reasons that caused a potential resurgence of nationalism after Tito. Milosovic and his other ethnic counterparts revived nationalist politics in his absence. The IMF forced the breakup of Yugoslavia's public sector, ending the shared socialist vision of the country. The economic prosperity of Tito's era ended. All this culminated in an abrupt end to national unity.

That seems to be bones of it. However it is a complex question, needing a lot more than a few paragraphs to fully understand.