Because developing good warplanes is extremely expensive and difficult. Also using same equipment as your allies has advantages - can secure external parts supp

Because developing good warplanes is extremely expensive and difficult. Also using same equipment as your allies has advantages - can secure external parts supply during a war, foreign planes can easily use your fields, etc. Of course, buying foreign planes comes with risks as well - parts supply might be embargoed, other countries will know something about your plane, etc. Judging by Norway's likely adversary (Russia), in all likelihood the risks were judged to be small compared to the advantages of using the best available plane from a country (US) usually not in good terms with Russia.