Here we go again.. There was no 'viking' people. Víkingr was the Old Norse term for people who journeys; pirates, raiders, adventurers, traders of any nationality. Or any religion; quite a few Scandinavian vikings were Christians, particularly by the 11th century. As for their self identities it was not entirely unlike today, in that there were Swedes (Svear), Danes, Norwegians, but also Geats (Götar), Gotlanders. But these were futher subdivided, as culture, traditions and laws varied within those groups, so one had for instance Jutes in Denmark, East and West Geats in present-day Sweden and Trønder in Norway.

The Scandinavians spoke a common language (with regional dialects) and had many common cultural traits. But as this recent thread explains in greater detail, there is extremely little we know for certain about Norse views of the afterlife. It's not known that they believed in any kind of reincarnation. We don't even know whether it makes sense to think they all had the same views.

From sources ranging from archaeology to place-names, there are many bits of evidence showing regional variations in how religion was practiced. And much of it is also at odds with what one might believe from the stories in the Eddas (which were written centuries later in Iceland). This is all particularly relevant with burial customs, as that is something archaeology can tell us about. And the results there are that burial customs in particular seem to have had great local variation. To quote a summary by professor Anders Andrén:

In some regions inhumations without visible markers dominate, in other areas mounds with cremations, and in still other districts stone settings with cremations. In a recent study of southern Scandinavia (Svanberg 2003) the regional burial customs are estimated to account for 90-95% of all graves, leaving only a thin strain of pan-Scandinavian burials such as the boat graves (Müller-Wille 1970). Since Old Norse religion was defined as a ‘way of life’, the diversity of rituals means that it cannot be perceived as a uniform and homogeneous religion. Instead the religion must be regarded as a series of partly overlapping traditions, differing from place to place and from time to time, and also between different age groups, sexes, and social groups. Perhaps the shared Scandinavian features, such as boat graves and sacral place-names, should primarily be viewed as the religious expressions confined to an aristocracy with wide-ranging connections all over Scandinavia.

So in other words, things like the great mounds with ship burials were common to kings across Scandinavia (and certainly eh Sagas and Eddas document frequent contact and intermarriage in those circles) and that likely reflects similar views on the afterlife. But that was only a tiny elite. The rest did not seem to bury their dead in the same ways (including those who were vikings). Overall, cremations was more common (among those given a ritual burial) compared to non-cremated pre-christian graves, but it was far from rare. The bigger difference is in other details though; did one build a mound/tumulus or not? Out of stones? Dirt? Both? Were those not cremated placed in a particular orientation, and if so which? What was done with ashes and bones between cremation and burial? And so forth. Nor did they memorialize their dead the same way. (e.g. most runestones in the world are memorials of dead people from 11th century Uppland and Södermanland in Sweden)

If there really was a single reason for why cremation was more popular, we don't know it. That reason/those beliefs may have changed with time as well, as cremation began in Scandinavia with the start of the pre-Roman Iron Age, 500 BCE. In other words, a time farther removed from the Viking Age than we are.

Sorry if this is a disappointing answer. A lot of answers about 'Viking' topic here end up being very elaborate ways of saying "we don't know". It's because there's quite a gap between what's really known and what people think we know about the Viking Age, as there's lots of popular books and TV and whatnot that basically take the Eddas and Sagas at face value and infer and extrapolate and speculate even more on top of that. But once you take a source-critical approach to those sources, as historians have for the last century, you realize how much of that castle is built on a foundation of sand.

Anders Andrén, Behind "Heathendom": Archaeological Studies of Old Norse Religion, Scottish Archaeological Journal Vol. 27, No. 2 (2005), pp. 105-138

Fredrik Svanberg, Decolonizing the Viking Age. 2, Death rituals in south-east Scandinavia AD 800–1000, Acta Archaeologica Lundensia. Series in 4° 24, 2003