Why haven't vending machines ever accepted pennies?

They have accepted pennies is the short answer, depending how far back you go with them.

This is an indication how far back they go:

The first recorded reference to a vending machine is found in the work of Hero of Alexandria, a first-century engineer and mathematician. His machine accepted a coin and then dispensed a fixed amount of holy water.[1][2] When the coin was deposited, it fell upon a pan attached to a lever. The lever opened up a valve which let some water flow out. The pan continued to tilt with the weight of the coin until it fell off, at which point a counter-weight would snap the lever back up and turn off the valve.

Despite this early precedent, vending machines had to wait for the Industrial Age before they came to prominence. The first modern coin-operated vending machines were introduced in London, England in the early 1880s, dispensing post cards. The first vending machine in the U.S. was built in 1888 by the Thomas Adams Gum Company, selling gum on train platforms. The idea of adding simple games to these machines as a further incentive to buy came in 1897 when the Pulver Manufacturing Company added small figures which would move around whenever somebody bought some gum from their machines.

https://en.wikipedia.org/wiki/Vending_machines#History

The vending machine industry has always been plagued by enterprising criminals who inserted slugs or relatively worthless foreign coins into machines in the time-honored tradition of trying to get something for nothing.

In the 1930s, a slug rejector was invented that could differentiate US coinage from Mexican centavos of the same size. The slug rejector worked by determining the metallic content of the coin. Although the slug rejector could easily tell the difference between silver or nickel and a slug, it could not tell the difference between a worthless token and the copper in a penny.

http://www.freepatentsonline.com/4111292.html

When I was a kid, vending machines absolutely took pennies. After all a soda was only 10¢ in the 1960s. So 10 pennies would make the whole purchase price.

If memory serves, vending machines accepted pennies all the way into the 1980s, but at that point vend items were starting to be above 50¢ so as was said before it would fill the coin box fast. I do remember on ocassion unloading extra pennies on the vending machines.

FYI back in the 1950s and early 1960s even parking meters took pennies. So did toll booth change baskets. Later on however they did stop accepting pennies because it was too time consuming to count and because people hated paying tolls and would always pay the full toll in pennies.

Like vending machines the coin baskets would still take nickels and dimes, but even then they upped it to only taking quarters, much like parking meters now.