This 2018 report by Ecofys und adelphi suggests that the original objective of the policy was to reach a certain market penetration and resulting network effects to kickstart electric vehicles.
[A] consensus was reached to uphold the financial incentives until at
least 2018, or until a total EV car stock of 50,000 vehicles is
reached. […] The fiscal incentives were intended to be upheld until
2017 or until the benchmark of 50,000 was reached (Zeniewski, 2017)
but have been prolonged. Some argue that the EV market in Norway is
now mature enough to gradually phase-out incentives, but the Norwegian
EV association and environmental organisations argue that the level of
support will need to increase to ensure a breakthrough of e-mobility
beyond early adopters and second or third cars.
The reason for a phaseout would hence be that the agreed objective of the policy has been achieved.
That said, to my knowledge the phaseout has yet to happen, see e.g. this article from January 2021.