To talk about this question, we need to rewind quite a ways from Pompey. Pompey was the at the end of a process which had been going on in the East since early 2nd century BCE. In the aftermath of the battle against the Seleucid Antiochos II in 190 BCE, the Romans laid bare an ugly truth: the Hellenistic kingdoms which had been built from the parcels of Alexander's empire were decaying, declining, disappearing, and in their place was a power vacuum. The Romans were, all things considered, quite slow in inserting themselves into this space. The massive territory they took from Antiochos II was left to the King of Pergamum to administer--in fact, the Romans seemed to have preferred "client kingdoms" in the East to the kind of aggressive and direct involvement they had utilized so often in their early expansion. In 133 BCE, the last king of Pergamum died without an heir and left his whole kingdom to the Roman people. Even still, the Romans were not exactly quick to involve themselves directly--at least not officially.

The Greek cities of the East were rich, despite centuries of milking by Hellenistic dynasts, but they were unable or unwilling to adapt to the realities of the post-Alexander world. Corinth and the Achaean League made a valiant attempt to pool their resources in the face of Roman power, but it wasn't enough, and after 146 BCE all the old centers of mainland Greece (Athens, Corinth, Argos, etc) were permanently under the Roman thumb. The Greek cities further east, however, were in a sort of limbo in the 2nd century. They had long been subject to the dynasts, but now found themselves precariously perched between Rome in the West, the emerging Parthians in the East, and the suddenly powerful and aggressive kingdom of Pontus in the North. They were ostensibly "free," though they all had obligations and treaties and other ties to one or the other or all three. There is a lot of inscriptional evidence to show what was going on in these numerous Eastern cities, and for the most part it is business as usual.

Roman intervention is often (rightly or wrongly) described as reactionary, and in the East that description holds some truth. Unofficially, Romans (actually, mostly non-Roman Italian businessmen) had been flooding into the Eastern cities since the early 2nd century BCE. There was money to be made in this rich, rudderless land, lots of money, and the business-class Romans/Italians had a nose for such things. The city of Priene was in Asia Minor (western coastal Turkey), just down the road from Ephesos and sharing a bay with the ancient Greek city of Miletos. We have a good deal of epigraphic evidence which shows how Roman business interests were in a quasi-legal battle with Priene's citizens over all sorts of economic clashes, including tax collection. In 104 BCE, Nicomedes III, king of of neighboring Bithynia, complained that Romans(/Italians) had been coming into his country and taking his subjects as slaves. After Pergamum was willed away to the Romans, there was an official province of Asia, and its borders steadily grew even as Roman/Italian speculators leaked out in every direction. Officially, however, the Romans were busy in the late 2nd century elsewhere, with Jugurtha and the marauding Celto-Germanic tribes.

This was more or less the situation when Mithridates VI of Pontus comes roaring onto the scene. He was young, dynamic, and fancied himself the new Alexander. He had been quietly expanding the kingdom of Pontus in the Black Sea region, and in 108-7 he and his neighbor Bithynia made a joint invasion of Paphlagonia. The Romans asked them to stop that, but they ignored the request. The Roman celebrity Marius didn't make his way to Asia until 99 or 98 BCE, at which point he met with Mithridates and famously (supposedly) admonished him either "to be as great as the Romans, or obey them" (Plutarch Marius 31.2-3). There are some other developments--boring tangles of minor dynasts in these Eastern kingdoms (even the Senate was apparently bored by the details of this or that ruler's claims). We can skip ahead. After a fortuitous alliance with Armenia, Mithridates invaded the Roman sphere of the East in 89-88, while they were busy with the great war against the Italian allies back in Italy. By the Summer of 88, he had occupied the Ionian cities of Asia Minor (including Magnesia, Ephesos, and Mytilene) and had massacred all the "Romans" he found lurking (probably mostly Italians, not Romans, but by 88 BCE there was no longer technically a distinction). "The rest is history." Lucius Cornelius Sulla, the hero of the Social War, was given a special command against this invasion, had it taken away by Marian supporters, marched his troops on Rome to get it back, then departed for the East and war. He would spend the next few years kicking the Pontic forces out of Greece, then Asia, burning Athens and looting the ancient sanctuaries of Greece in the process.

The point of that long tale is the suggest that Roman involvement in the East was not exactly "attractive" at the start. It is certainly possible to frame an argument in which the Romans were very reluctant to screw with Asia at all, until a reasonably well-equipped and dangerous force came on the scene and killed Roman speculators. It was really Sulla that exposed the true wealth of the Eastern cities, and the opportunities for organized plunder they offered. Sulla, then Lucullus, and then Pompey would spend a lot of time and energy on figuring out ways to organize the East in a manner most efficient for transferring that wealth into Roman coffers. There was also, of course, a military component: Roman statesmen needed wars to fight, needed to have triumphal parades, and needed to defray costs with good old-fashioned plunder. Mithridates VI provided that, and after him the Parthians loomed as an opportunity (Crassus reached for it, of course, and Marcus Antonius had similar plans).

There is a fairly new monograph on Mithridates VI by Højte (2009), Mithridates VI and the Pontic Kingdom (Aarhus U Press). In particular, there is an article in there which explores this liminal period, "The Ambitions of Mithridates VI: Hellenistic Kingship and Modern Interpretations" by JM Madsen which is a really interesting read.