Until the 1950s, there was no real need to define ‘draught beer’: it was the opposite of bottled beer, simple as that. Then keg beer came along (Watney kegged bitter in 1936; Flowers coined the term ‘keg’ in 1955) and suddenly draught beer had a split personality.
For many people, it didn’t matter. As long as they got a ‘pint’, they weren’t fussy about where it came from. Some ‘connoisseurs’, however, knew they didn’t like keg, but
weren’t sure exactly needed a new term to describe exactly what it was they did like.
They tried ‘beer from the wood’ (in common since at least the turn of the century), until some smart arses pointed out that most casks were made of metal these days anyway. While the confusion continued, big brewers happily promoted keg beers as good, traditional, draught made the way it always has been, from premium malt and hops, only slightly better.
The Society for the Preservation of Beers from the Wood decided the answer was to reclaim ‘draught beer’ and lobbied government for several years from the late sixties. It was a con, they argued, to call keg bitter draught. Draught, they said, was, you know, proper draught, the good stuff, from the wood, but not necessarily actually from wood… oh, sod it. They were repeatedly rebuffed by Whitehall.
In 1969, the Monopolies Commission, which had been investigating various industries in the great era of corporate mergers, reported on pubs and brewing (link to PDF). As bureaucrats are often required to do, they spent no little time establishing terminology, and came up with this handy guide:
We use the description ‘draught’ beer to include any beer which is supplied to the retailer in bulk containers and drawn to order in the pub for each customer. All the large brewers and many smaller ones now brew a kind of draught beer which has become known as ‘keg’ beer. Although the word ‘draught’ is sometimes used to distinguish traditional draught from keg beer, for the purposes of this report we call the former ‘cask’ beer. [B&B's emphasis.]
The report, which was widely read by those with an interest in beer, probably did a great deal to popularise the use of the term ‘cask’ in this way.
The report, if you’ve got the patience, is a fascinating read, especially the opening section which summarises the types of beer commonly available and most popular with drinkers.
UPDATE: worth noting, too, that Frank Baillie’s 1973 The Beer Drinker’s Companion classifies each brewery’s beers as either draught, keg or bottled.