It was conquest! Always conquest.

The Austrian Empire, or the Austro-Hungarian Empire, had its roots with the Habsburgs, and the Duchy of Austria. Austria and Vienna lay on trade roots coming east out of Italy, North out of the Balkans, and South and East out of the Baltics/Ukrainian steppe. Throughout the Renaissance and Early Modern period, this made Austria wealthy and powerful. The Habsburgs, Austria's ruling family, also gained tremendous political power across most of Germany, they became the Emperor of all of Germany. Austria used this power and prestige to spread their rule across southern and eastern Germany. They joined the thrones of Bohemia and Hungary to the Austrian one, giving Austria de facto rule over the area, while northern Italy was drug into Austria through a series of wars, and an advantageous marriage in Spain, which for a time unified the Austrian and Spanish thrones.

But all this power, wealth, and prestige also made Austria a target. It waged several notable campaigns against the Ottoman Turks, who after toppling the Byzantines continued to push north through the Balkans. Austria continued to strengthen its southern border, as well as Hungary's southern border, to prevent serious Turkish invasions. Thus Austria began to see itself as the traditional defender of Christianity in the Balkans, as well as the defender of Europe against the Islamic Turks.

So really, Austria, and later Austro-Hungary, was a polyglot empire because Austria expanded in such a haphazard and seemingly random way. The most important connecting thread was always "what is best for the Habsburgs?" who sat at the top of the whole pyramid. In the Renaissance, it made Austria a dynamic political and military power, which hung a curtain across southern Germany. But following the Thirty Years Wars, and especially after the Napoleonic Wars, the multitude of nationalities, religions, political and social ideas, and ethnic alignments all made Austria a strange and unhealthy nation. The Habsburgs continued to rule as if the Austrian Empire were still the "Habsburg Lands", which you sometimes see printed on older maps of the Renaissance. This led to the breakdown of Austria, which really culminated in the July Crisis of 1914, which directly caused the conflict which would destroy the Austrian (at that point Austro-Hungarian) Empire in 1918.

Are you talking about the 16th century event of "unification" under a Habsburg monarch, or the event of the 19th century organisation of the Empire into Austria-Hungary? Or maybe are you asking in general what was the reason why so many different nations stayed together for so long?

(I just want to here mention that i can't speak for the Bohemian part of the Empire, but mostly for the Croatian and i suspect Hungarian part.)

Well anyway, generally speaking one of the main reasons is the threat of the Ottoman Empire. After the Battle of Mohacs in 1526 Turks conquered a large part of the then independant Kingdom of Hungary, Bohemia and Croatia and the last Hungarian king died without an heir. With the prospect of the Ottoman (Muslim) conqest, the nobles in the wesern parts of the Hungarian Kingdom elected Ferdinand Habsburg of neighbouring Austria (later Holy Roman Emperor) as the king to protect them. (Not to say that all Hungarians chose Habsburgs over the Ottomans. Quite the opposite, many supported the Ottoman backed Hungarian king in a kind of civil war)

What followed were several centuries of fierce continuous wars, skirsmishes and border conflicts between the Habsburgs and the Ottomans for the area. For the nations on the borders, the Empire's protection was desperatly needed despite all the bad things it brought with it, and as such few attempts of splitting up were ever considered.

It lasted until around beginning of 18th century (e.g. we can take the marking point the 1699 treaty of Karlowitz) when the Ottoman threat was almost gone and most of the areas of old kingdom back in Habsburg hands, and by then we already have a well established state that the strict feudal system of law and inheritance made de jure right of Habsburgs which few had the power, legality or will to attempt to dissolve.

Since then fast forward couple of decades and with Enlightment, Apsolutism, rise of citizenry and later nationalism, Napoleon, ( and with the outside threat of Ottoman Empire gone) the Empire entered into a series of years of political turmoil, which eventually resulted in formation of Austro-Hungary dual empire. Still then and there, as you noticed, were not so many common factors between the nations in the Empire and I would say the long tradition of the Empire had been the main reason why no main political party thought about dissolution of the empire (together with the fear of being fiercly prosecuted for the mention of it of course) It needed a world war to finally break it.