Because Buffalo is a small town, and an authentic underdog. Despite the cliches, the unhappy fact is that American society hates the poor, the weak, the underdog. It worships strength and success. The only time an underdog gets affection is when it wins, a paradox of the first order. That sportswriters and others try to deny this only makes it more nauseating.
All the "lovable loser" franchises come from larger metro areas (Chicago, Boston, NY) which can already claim fans, money, and success. When a franchise in a smaller city loses (Kansas City in MLB, Green Bay during the 1970s and much of the 1980s, for example) the world simply ignores them, or sportswriters mock them.