>> what are those reasons? - do you mean in the books - or by the relevant archaeolgists?

Both to an extent, depending where you are talking about. In Germany for instance the stones are only just becoming an acceptable subject. There was (is in places) still a Nazi association with them. The Dutch Hunnebeden-type monuments of Drenthe actually spread over the German border, but you never read of them for instance. There are a load of tombs and stones in Hesse too. Places like Caucasia people just haven't bothered until recently. I remember Annexus saying that in parts of Spain and Portugal he just had to go around asking shepherds if they knew of any monuments locally, because no one had really bothered looking there before. The Austrian monuments have been recorded, but not greatly publicised outside of Austria. The list goes on and on.