Oh, we have heaps of parties - there are at least twenty that are recognized on ballots; the problem is only two of them actually have a realistic chance of winning most of the time.
I see, and I suppose it is practically impossible for other parties to have a real chance. I obviously don't know how it is there, but I am going to make a guess that it is kind of a collective perception that you really only have two options. Changing this would probably be nearly impossible if it has become the accepted norm.
It's hard to know for sure, but I suspect advertising might play a large part in this. The two big parties probably sit on the big money, so they can afford a huge advertising campaign. If most people are exposed to mostly ads from only two parties, they will probably only consider those two. Political advertising used to be completely forbidden in Norway, but after a ruling in the EU, we had to allow some political ads.
Currently, political ads are banned on regular TV (which hardly anyone watch anymore), but it is allowed on any other form of media. Thankfully, as a nation, we still view political ads largely negatively, so we only have political ads a couple of months before elections. Usually these ads are in the form of pamphlets in your mailbox. I have heard that political ads are being served on social media as well, but since I don't frequent social media I wouldn't know. Besides I always use adblock.
On a personal level, I strongly support the notion that we need multiple parties. Even further, I believe governments should always consist of more than one party. This is a good way to ensure that more than one view is represented in any ruling matter. The last time we had a single party government in Norway was 2001, since then it has always been coalition governments.