Tried to get Ryanair interested in flying to Trondheim since 97. They shy away from the Norwegian market because all the main airports are owned by the government and they can't get a deal, which is what they feel is essential for their model..They have done international traffic to a couple of alternative privately owned Oslo airports, the other cities don't have alternatives, but they are far away with more inconvenient ground links so are not as popular among Norwegians and therefore no god for intra Norwegian flights. As for easyJet even though their offering is more comparable to Norwegian they where never that active in Norway, probably seeing the total population as to small forgetting its also about wealth, distances and poor alternatives. There might be no ocean between Oslo and Trondheim but the alternative to a 45 min flight is/was a 7 hour drive at 60-80 km/hr on a lot of very boring single lane highway. Same for Oslo-Bergen, and Oslo-Tromsø is a multi day journey.
This was the way Norwegian grew so fast from nothing. They made money on these and other scandinavian trunk routes that they then spent on an ambitious expansion, unfortunately without creating a bit of a financial safety buffer in the process.