Because on the face of it, Pale Ale is very obviously a Beer, with all that hopping. I think the reason it wasn't may be hydrometer-related.
"Ale. Ala. Barley Wine. Cervisia, or Ale and Beer, are prepared by the fermentation of an infusion of Malt and Hops. Pliny speaks of drinks made from corn, and says, that zythum is made in Egypt, Celia and Ceria are made in Spain, and Cervisia andmany other sorts of drink are made from different kinds of corn in Gaul.
Ale is prepared from pale malt, or that which has been lightly dried. There are many varieties, all of which usually contain more saccharine matter and mucilage than Beer, or Porter; but some varieties do not contain any larger proportions of those ingredients than are usually found in Beer. The Old Burton and Scotch East India, are so thoroughly fermented that most of the sugar has disappeared. The Pale, or Bitter, or East India variety contains a larger per centage than the usual amount of Hops, and hence is frequently prescribed for invalids and convalescents.
New, or Mild ale abounds in undecomposed sugar, making it more nutritious but less alcoholic than the more thoroughly fermented varieties.
In brewing Ale, pale, or lightly dried hops are selected, and then the process of brewing (See Brewing) is to be conducted in the usual manner. Almost every locality, where Ale is manufactured, yields a liquor possessing some peculiarity, owing to the malt or water used, or to the peculiar method of brewing adopted. Ordinary Ale contains from five to six per cent of Alcohol. The stronger varieties contain about eight per cent.
Barnstable Ale. This is prepared by a particular manner of. treating the mash first to boiling and then to cold water. It has no very peculiar qualities as a medicine.
Bavarian Ale is fermented at a low temperature in wide, open, shallow vessels, so as to expose the wort freely to the action of the oxygen of the atmosphere. The temperature of the air where the fermentation takes place should not exceed 45° or 50° Fahr., and hence it is usually manufactured in the winter.
Burton Ale is one of the strong varieties, and in chemical composition and therapeutical properties is closely allied to some of the Wines. It has often been called "the Wine of Malt."
Dorchester Ale is a mild variety."
"Journal of Rational Medicine, Volume 2", 1861, page 139.
See what it said: "Ale is prepared from pale malt." I've seen this mentioned before in 19th-century sources. I think it came about because the most famous type of Beer, Porter, was (along with Stout) the only dark Malt liquor.
It's all the fault of thr French and the hydrometer. When brewers started using the hydrometer, they immediately noticed how much better the yield was from pale malt than from darker base malts. Things like Brown Ale disappeared, as brewers switched to usung pale malt as the base malt in all their products.
With Ale becoming associated with a pale colour, it's only logical that you wouldmn't call Pale Ale a Beer, because beer was dark. Make sense?
Bavarian Ale. That made me smile. It's the sort of crap the BJCP would come up with.