Local, small scale trials of LHV's (Longer and Heavier Vehicles aka Gigaliners, EuroCombi, EcoTrucks or MegaTrucks) have been running already for several years within many states of Germany (link with details) but what's been blocked is movement between the states.  

Trucks of this size have been running in Sweden and Finland since the 1970's. Similarly in Canada, another country highly dependent on forestry, LCV's (Longer Combination Vehicles) are permitted. Depending on the combination, they can go up to 25 meters and 62.5 tonnes total weight, very similar dimensionally to the European trucks (25.25 m, 60 t) but using a much different configuration.

Efforts to allow national use of Canadian style LCV's in the US (they're permitted in some states) have been routinely defeated by the railroad and highway safety lobbies on safety grounds using arguments very similiar to those raised in Germany. Ironically, since deregulation at least (1980), this has helped to drive down truck driver pay (more trucks needed for a given amount of freight) as this is the one key cost that trucking companies control. This in turn has forced drivers to work longer hours, often in violation of hours of serivce limits, leading to an increase fatigue related acidents (here's a link on the issue in general).

Driver pay and working conditions are generally better in Europe than the US, but the industry is moving down a slippery slope thanks to the opening of borders. To me it's no coincidence that the countries that are most at risk of losing driver jobs and haulage companies to the east are also the ones that are most supportive of the LHV's. The restrictions put in place on driver training and qualifications become an effective barrier to entry while the improved economics allow the companies to continue to compete despite a higher cost base.