There seems to be two reasons. First, as you surmised, is cost to the Norwegian public coffers coupled with a desire for commercial independence and sustainability. Emphasis added, and did not use the FT as primary source because of paywall issues.
The cost of these subsidies is becoming contentious as cited by quotes from politicians in Norway as reported by the Financial Times. The paper prepared an extensive article on the subsidy issue versus the dream of shifting Norway’s car fleet away from fossil fuels.
As Andreas Halse, the environmental spokesman in Oslo for the opposition Labour Party, said, “What we have proven in Norway is that if you give enough subsidies and impose enough restrictions on fossil fuel vehicles, people will buy electric.” He went on to say, “If we want to continue to be an example for the rest of the world, we need to show how this can be commercial. We need to get there because we can’t rely on public finances forever.”
But another reason should be added to the mix
Norway plans to trim lavish tax breaks for Tesla and other electric cars that have given it the world’s highest rate of battery-vehicle ownership, the right-wing government proposed on Thursday.
The draft 2018 budget would mainly affect large cars weighing more than two tons, it said. Norwegian media dubbed the changes a “Tesla Tax”, intended to cut down on sales of luxury models such as Tesla’s Model X sport utility vehicle.
Norway seemingly doesn't want to be giving tax breaks to the affluent, or at least not subsidizing the purchase and usage of affluent cars.