Given the supposed prior long history of use, a rather trivial question might be why it took till the 16th century for juniper berries to finally become an entrenched spirits ingredient. (The 8th century was apparently the advent of accomplished distilling.)
The history and folklore concerning the juniper tree is long reaching. The first recorded mention of use is in an Egyptian papyrus from 1500 B.C.E. Juniper was the symbol of the Canaanites fertility goddess Ashera. Western European folklore tells that is a juniper tree is planted by the door to your home, a witch cannot enter. Juniper incense has also been used by the Scottish to ward off the evil eye, and by the Tibetans to remove demons.
The purple, blue, violet, or blackish-brown fruits are harvested in early autumn for culinary and medicinal use. Herbal uses of Juniper Berry dates back to early Greek and Arabian physicians. During the Bubonic Plague, cautious people kept a few berries in their mouths to produce an antiseptic aura and prevent infection. Juniper tea was once used to disinfect surgeon's tools. Juniper Berries were also used as a food and a medicine by the Indians of the American plains. Historically, many conditions have been treated with Juniper Berries by several cultures, including gout, warts and skin growths, cancer, upset stomach, and various urinary tract and kidney diseases.
Few gin distillers maker their own alcohol; gin usually starts with neutral spirit, a commodity that gin distillers buy in bulk. While gin must have a strong juniper flavor to it, distillers are free to add any other botanicals they like to achieve their target taste. Citrus, nuts, and spices all commonly find their way into gin recipes. This contributes to making each gin different. Juniper berries are picked wild by independent workers throughout Europe. To battle malaria, bitter quinine tonic water was mixed with gin to make it more palatable; according to legend, this is how "the gin and tonic" was born.
"The juniper tree beside my new house in Columbia County, N.Y., is loaded with blue berries. When is the best time to harvest them?" --> Possibly never. Of the roughly 40 species of juniper, a small number are poisonous and a majority have bitter fruits. Only a few yield edible berries (actually modified cones) and only one is routinely used for flavoring. (Juniper Berries? Be Picky)
There had been prior evidence of crudely distilled alcoholic beverages, liquors made from things like rice and mare’s milk in Asia as far back as 800 B.C. [...] evidence of a knowledge of distilling that found its way to Ancient Greece and continued into the first century A.D. Writings in the 4th century A.D. also attribute the development of the tribikos—or three-armed pot still—to “Maria the Jewess,” the first documented Western alchemist. (Whether she actually invented it is unclear.) But it wasn’t until the 8th century A.D. that Arabic alchemist Abu Musa Jabir ibn Hayyan designed the alembic pot still, a contraption that allowed for the effective distillation of alcohol. --The History of Distilling
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