Why is happy hour called happy hour? It could be something like “Getting an Alcohol dosage time” or it could be something like “Wanting to drown myself in alcohol at a lower cost time”. Then, I tried searching for it at Wikipedia. The term Happy Hour orginated in the US Navy in 1920′s where the term itself is a navy slang for on-ship performances.
Now, if you think that there’s no history why happy hour should always be early, then there’s also a historical explanation for that. Volstead Act, which was passed by the US Senate in 1919, enforces the prohibition of alcohol (the intoxicating beverage and not the one that kills 99.99 percent of germs). So the happy hour was held at underground drinking establishments and it should be before dinner time. The general slang term they used for that (the establishment) is speakeasy.
All the while I thought that Happy Hour was just a strategy for bars to sell more beers at a lower cost during hours where people are not yet jam-packed since bars usually expects more people later in the evening.