There is no reason, beyond customer expectation, why 'mild ale' should be any weaker or stronger than 'bitter'. If you want to be precise about this, 'ale' was originally made without hops for the simple reason that we didn't have any! 
Brewers added all sorts of bitter stuff, like spruce resin, to improve the flavour but it still wasn't very nice. The drink with hops in it was, and still is, called beer.
Over time the distinction was lost because nobody was making unhopped ale anymore - but this didn't just happen overnight. There were many arguments between ale and beer brewers and there was a time when the ale brewers succeeded in getting the hop banned as "an evil and pernicious weed" - as befits a close botanical relative of cannabis!
Once beer became the norm there remained the question of exactly how many hops to use. In an age when brewing was still an art - and the chemical processes involved were largely unknown - the type of beer you could make was heavily influenced by the local water (or liquor, as brewers like to call it). Burton on Trent had water rich in sulphates and this was good for making bitter beer. London water was loaded with chlorides and carbonates and was better suited to milds and stouts.
In general bitters, especially the mis-named India Pale Ale, were more highly prized while mild 'ales' were the poor man's drink. Weight for weight, hops are the most expensive ingredient in beer so you wouldn't want to waste them in anything but your best brew. Furthermore, a strong beer required lots of malt and this in turn meant lots of hops to balance the flavour. Weaker beers could be made with fewer hops.
The idea that "bitter is best" still persists. There are many beer drinkers who, upon entering an unfamiliar pub, will instinctively ask for 'a pint of best bitter'. 'Mild' is often considered to be a synonym for 'weak' and, apart from a well known Irish variety, stout doesn't fair much better. Small beer, a weak drink made from malt leftovers, was also mild.
But there are exceptions. Theakstons Old Peculiar is a high quality mild ale, as is the world famous Newcastle Brown Ale - well it was once! Most barley wines are also not particularly bitter.
Brewers added all sorts of bitter stuff, like spruce resin, to improve the flavour but it still wasn't very nice. The drink with hops in it was, and still is, called beer.Over time the distinction was lost because nobody was making unhopped ale anymore - but this didn't just happen overnight. There were many arguments between ale and beer brewers and there was a time when the ale brewers succeeded in getting the hop banned as "an evil and pernicious weed" - as befits a close botanical relative of cannabis!
Once beer became the norm there remained the question of exactly how many hops to use. In an age when brewing was still an art - and the chemical processes involved were largely unknown - the type of beer you could make was heavily influenced by the local water (or liquor, as brewers like to call it). Burton on Trent had water rich in sulphates and this was good for making bitter beer. London water was loaded with chlorides and carbonates and was better suited to milds and stouts.
In general bitters, especially the mis-named India Pale Ale, were more highly prized while mild 'ales' were the poor man's drink. Weight for weight, hops are the most expensive ingredient in beer so you wouldn't want to waste them in anything but your best brew. Furthermore, a strong beer required lots of malt and this in turn meant lots of hops to balance the flavour. Weaker beers could be made with fewer hops.
The idea that "bitter is best" still persists. There are many beer drinkers who, upon entering an unfamiliar pub, will instinctively ask for 'a pint of best bitter'. 'Mild' is often considered to be a synonym for 'weak' and, apart from a well known Irish variety, stout doesn't fair much better. Small beer, a weak drink made from malt leftovers, was also mild.
But there are exceptions. Theakstons Old Peculiar is a high quality mild ale, as is the world famous Newcastle Brown Ale - well it was once!
Most barley wines are also not particularly bitter.
