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. The first dark hours return in late August, so the season is under way in early September; by mid-September the whole evening is dark.
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.