That's because Strangford Lough Brewing Company actually care about the beer, while Arran More appear to be a Beer Brand, where the beer itself is secondary to the marketing.
The bottle label waxes lyrical about the history of the Island and the “Secret Essence of Arran More” but it tells you nothing about the beer. This is a brand with a beer attached, not a beer with a brand name.
I think the fact that the Arran More glass is too small for a bottle of Arran More beer, is very telling. What brewer would allow that to happen? It's obvious that they just wanted a beer glass with their logo on it and gave no thought to how it would suit the beer.
I have heard vague fairy stories about the possibility of them opening a brewery on the island, but that's just not feasible. Breweries use vast amounts of water and Arran More already import a significant portion of their drinking water from the mainland.
Also note that their office is actually in Burtonport, which is on the mainland, not on the Island.