Well, it's debatable as to whether the papacy really had much to do with it. Little pockets of disaffected Normans had been fighting as mercenaries in the region for decades (Loud, 'How "Norman" was the Norman Conquest of Southern Italy?', Nottingham Medieval Studies 25) but the big Norman assault on Sicily itself was really a Byzantine assault. As Hubert Houben succinctly observes, 'when a brother of the [Islamic] Emir of Palermo incited an uprising, the ruler called on the Byzantines for help' and the Hauteville Normans were at that time on the Byzantine payroll as mercenaries. When the commander of the Byzantine forces was recalled, the Normans took over the enterprise on their own account. (Houben, Roger II of Sicily: a Ruler between East and West). Presumably the pope was glad to see some Latin Christians having a stab at reclaiming the island, rather than the armies of the Greek Orthodox Byzantine Empire, but it wasn't instigated by the church.

The Hautevilles who left Normandy did so because the family was impoverished by aristocratic standards. Tancred de Hauteville couldn't support all of this sons in a suitable manner and there was nothing much for them to look forward to in terms of social standing or what we'd now call 'career' - 'their own neighbourhood would not be big enough for them' and 'individual shares [of their inheritance] would simply not be big enough' (Geoffrey Malaterra, The Deeds of Count Roger of Calabria and Sicily and of Duke Rober Guiscard his Brother, trans. G. Loud). Robert Guiscard and his kin went to Italy to seek their fortune, to try and carve out some sort of meaningful and productive life for themselves. They had initially fought alongside the Byzantines and then turned on them to capture Byzantine cities in southern Italy as soon as it became practical to so, culminating in stamping out the last of the official imperial presence with the conquest of Bari in 1071 (Oldfield, City and Community in Norman Italy).

There are occasional references to religious righteousness as a motive but they're not enormously convincing. Geoffrey Malaterra's roughly contemporary account mentions that Roger I was suddenly aflame with a desire to conquer Sicily when he heard it was occupied by 'unbelievers' but then remarks in the very next sentence that he was 'always eager for conquest' (Malaterra, Deeds) so I think Malaterra's reading of the situation is much the same as mine - religion was a nice bonus justification, but not the real point.

Better yet, 13th century Arabic chronicler Ibn al-Athir attributes to Roger the pious exclamation 'the lands will become Christian lands' at the genesis of his conquest of Sicily, but impedes the earnestness of this a little bit by following it with 'Roger raised his leg and gave a loud fart' (The Chronicle of Ibn al-Athir for the Crusading Period from al-Kamil fi'l-Ta'rikh, Part 1, trans. D.S. Richards). I assume the flatulence here is a gesture of contempt for the Islamic presence on the island, but I think it's quite telling that this is one of the few primary sources I know of that attributes much religious conviction to the conquest, and that's all it has to say on the matter. Regardless of what the pope may or may not have thought of the situation, the motive of the Hautevilles themselves was clearly far more material and self-serving.

Even if there was stronger papal endorsement than I'm aware of, religious conviction wouldn't be the only motive on the church's part either. Look at the way a series of popes tried to neuter Sicily once the Normans held it. It was fine as long as Norman ambition remained modest but once Roger II started trying to name himself king, the papacy wouldn't stand for it. Not only did the popes repeatedly evade and backpedal on investing Roger with the kindom of Sicily, Pope Innocent II actively incited war against Roger when he encouraged King Lothar III to march his troops all the way down from (present day) Germany to drive out Roger's occupation of mainland southern Italy and push him to back to Sicily proper in 1137 (Houben, Roger II). Ostensibly the pope was concerned about the ungodliness of Roger's realm and wanted to free Italy from his fallacious grasp, but this is a pretty thin excuse considering how devout Roger clearly was. Not only did he spend half his life trying to get papal recognition of his kingdom because it mattered to him to be a godly ruler, but reportedly he also founded the cathedral at Cefalu in response to a vision he had while almost drowing at sea. Witness, too, the splendour of the Cappella Palatina (photo mine, taken June 2013), the chapel that Roger had constructed within the royal palace at Palermo - a suitably grand medieval display of devotion to God, and at the same time a convenient location for diligent worship right on Roger's doorstep.

In light of Roger's piety and commitment to godliness, it seems hugely improbable that the pope was genuinely concerned about a heathen lurking in the south. Admittedly the strong presence of Arabic and Greek citizens remaining in Sicily from previous occupations, and Roger's own proclivity for appearing in public dressed in Islamic garb, probably aggravated the church but I have my doubts that this alone would be enough for Pope Innocent II to have King Lothar III's armies come all the way from Germany to take Roger's mainland territories from him by force. More likely this was a handy pretext for, and welcome side effect of, curtailing a growing secular power in Italy.

Positioned as it is on both the north-south and west-east travel routes through southern Europe, Sicily was a prime stopping point for travel through the Mediterranean. If you were journeying between northern/western Europe and Africa/the Holy Land, you would usually stop off at Sicily. Additionally, the anti-clockwise nature of the winds in the Mediterranean meant that in the age of sail Sicily was an even more necessary waypoint for travel in that region - anyone going east to west would have to head along the northern edge of the Mediterranean and thus directly to Sicily (Bennett, 'Norman Naval Activity in the Mediterranean c.1060-c.1108', Anglo-Norman Studies 15). It was simply the most practical of the Mediterranean ports.

My point is that if there was active papal endorsement of the Norman invasion of Sicily and it was purely religious, then I think there would have been far less hostility between the subsequent popes and the Siculo-Normans than there actually was. In fact, the papacy just continued to fling military might and ecclesiastical sanctions at Sicily once the Normans were established there, so I'd argue that curtailing the growth of a power bloc that could rival the Vatican, whether that bloc was Christian or Muslim, was the main motive on the part of the papacy in any attacks that they either conducted or condoned, or least an equal motive alongside ousting Islam from the island. I wouldn't be surprised if there was some resentment that rather than cleansing Sicily of non-Christians the Normans kept a lot of them around, but I doubt that was the primary reason for the fractious relationship of the following decades. No, the papacy also wanted to limit their potential rival(s) in the south of Italy, there's little room for doubt on that score. Combine that with the absence of significant evidence that the papacy was strongly involved in motivating the conquest, and the high probability that the Normans' own motives were likely mostly materialistic, and on balance the whole thing was largely about land and power with at best a vague, albeit probably sincere, strand of holy righteousness very much taking second place.