Ordinary or Best?

I've been rereading Andrew Campbell's Book of Beer in the last couple of days, a fascinating glimpse into not just beer and pubs but British society in the mid-1950's.

One thing that struck me is his description of Fuller's London Pride as "an excellent strong pale ale". Today, London Pride (OG 1040.5, abv 4.1%) is Fuller's standard bitter, in between the lighter Chiswick Bitter (OG 1034.5, abv 3.5%) and the stronger ESB (OG 1054, abv 5.5%).

Comparing the 1983 and 2011 Good Beer Guides confirms the trend towards regarding the best bitter as the standard beer. In my hometown of Stockport, the local brewery Robinson's biggest seller is its Unicorn Bitter (OG 1041, abv 4.2%). In 1983, the same beer was called Robinson's Best Bitter - their standard Bitter then (OG 1035) has become the rarely seen Old Stockport.

Similarly, Marstons Pedigree (OG 1043, 4.5%) has become a nationally avaliable flagship beer for its brewers, far outstripping their Burton Bitter (OG 1037, abv 3.8%). Only Young's still seem to be distinguishing their bitter and best bitters with Young's Bitter (OG 1036, abv 3.7%) and Young's Special (OG 1044, abv 4.5%).

So how and why did a pint of best become a pint of bitter?