Most Bronze Age burial cairns are located on heights. They share this tendency with burial sites from ages both before and after.
The reasons for their location can be many, and include the lack of viable flat land for construction due to nonexistent ditching in the marsh-ridden lands of prehistoric Sweden, as well as, in some cases, the geological uplift caused by the post-glacial rebound. Furthermore, one cannot rule out the possibility that burials in flat terrain have been equally common, but that these grave sites have been farmed away due to the superior usability of flat ground in farming.
Even should this be the case however, the hills, hillocks, knolls and ridges that nowadays house the majority of Swedish burial grounds seem to be part of a larger, global (one might even say universal) tradition of holy sites being constructed upon heights.
In part, this should probably be linked to our tendency to place the gods in the skies or on distant mountain tops - always aloof and impossible to reach but for the occasional hero or scoundrel of myth. From their perches high above the world, the divine beings could look out over their domains and devise their plans for Mankind. Thus, it stands to reason that graves constructed to prepare the deceased for the afterlife and the encounter with the Divine, would be placed as near as possible to the heavens (i. e. on topographical heights) .
As religion and society became ever more hierarchichal, one can guess that the political and religious elites began to demand burials that were based upon their status in society. As a consequence, the burial monuments of the high and mighty began to ever more resemble the sacred mountainous abodes of the gods. Such was the case with the Pyramids, the Grave of the Qin Emperor, and the more modest but still impressive Great Mounds at Upsala. And this was mirrored in the way religion evolved, as gradually a mirror image of the Earthly hierarchies were placed upon the Heavenly realm, where the supreme deity became ever more supreme and His subjects ever more downtrodden.