Serbia isnt a major nation
Does firearm tech and performance depend on which nation produced it and/or adopted it ?
if that's the case Austia also isn't a major nation but their AUG literally brought bullpup config into mainstream,
certain plastic salesman from same state who used to sell curtain rods made polymer framed pistol of his own and branded behind his name and look how it changed contemporary pistol market that even hundreds of years old sleuths had to bring their own designs in competition
There are lots of factors behind 6.x mm adoption hesitation;
one major factor being proliferation of 5.56x45mm, 7.62x39mm etc weapons that would take lots of monies to replace,
and yet Serbians are investing in it for good future,
also don't forget our major nation India had these tall ambitions to make its own assault rifle that had AUG like quick change barrels, polymer magazines so a soldier could look how much ammo is there in it and common rifle platform to replace all different class of weapons with,
and did a very average job about it,
finally importing "NEW" russian rifles to replace it
Serbians OTOH have been mostly
aatmanirbhar in that regard heck exporting their arms in international market in huge numbers already...