Three things need to line up: geomagnetic activity, a dark sky and clear weather. Northern Norway covers the first one most of the year — the auroral oval sits roughly over Tromsø, Alta, Lofoten and Senja, so even moderate Kp values can give visible aurora. That part is easy.
The dark-sky requirement is what makes the season so narrow. From late May to mid-July, places north of the Arctic Circle have the midnight sun — the sun never sets, the sky never gets dark, and even strong aurora is invisible. From late August things start getting dark again; by mid-September the season is open.
Weather is the wildcard. Coastal Norway (Tromsø, Lofoten, Bodø, Bergen) sees fast-changing cloud cover from the Atlantic. Inland Finnmark (Alta, Kautokeino) tends to be drier and clearer in winter. A clear night plus a Kp of 3 will often beat a cloudy night plus a Kp of 6.