The important thing about these areas is that they are mountain areas in the middle of the country. Which means that these areas are not accessible by sea. In the Viking age, basically all long-distance travelling happened using ships. This included movement of goods, troops and most importantly information. This leads to multiple factors which are important to answer this question. Firstly, this makes these mountainous regions hard to subjugate for anyone interesting in taking control of the areas. Since there is no good infrastructure at all to move your troops and the terrain is mostly mountainous, often with large increase in altitude this makes it hard to send anyone to actually take control of the area. This also brings us to my second point. As these areas where both hard to get too and mostly mountains anyways living here would be extremely hard if not impossible. Most of the area would be uninhabitable, mainly because it is over the tree limit and therefore there would be no wood for building houses or for making fires. The other reason for which it would be uninhabitable is because it is impossible to grow crops there during most, if not all of the year, because of snow and generally small amount of soil to farm in. The area that is not to high up to be farmed would mainly consist of valleys with steep sides, again mostly impossible to farm. In addition, its position away from the sea made transporting goods too and from the area costly and dangerous.

Because of this we can conclude that these areas where mostly uninhabited and even if people lived here the people living here would be poor. This combined with the fact that taking control over these areas would be hard, and definitively not worth it for the rulers, as there is little these regions could contribute because of the problems with transporting troops and resources from the area. It is also important to note that in Viking age Norway an important part of controlling an area is to actually be present in the area with your retinue and show that you actually control the area, therefore it would (once again) be really hard to keep control over the area as there is no infrastructure to move your troops.

As a final not I would like to comment a little bit on the Wikipedia article you linked. This is because, to me it seems like it has some problems with the way it presents the information. The unification of Norway is a quite complex event and an event in which there is many possible ways it could have happened. For instance, some historians believe that rather than starting his conquest in the east of the country he started in the western parts of the country. The Wikipedia article does not address this and it also seems to get all its information from Snorre Sturlason’s Heimskringla and therefore ignoring that both other literary sources and archeological findings disagree with both the order of the events and also the speed of the process of unifying Norway. I am not saying that the article is wrong, but rather that it does only present one of many theories around the unification of Norway and if you want to learn more about it, you should try to find additional sources that could give more information on the other theories.

Thanks for reading and i hoped i managed to answer your question. Sorry for my bad english and i hope it is not too hard to understand. If you do have any questions please ask.