It's no accident that the Osborne brothers are legendary

This well plagiarized paragraph is probably the closest account I will ever be able to find as to the origins of the Osborne Brother's fabulous career.

In June 1949, at age 17, Bobby Osborne made his first radio broadcast appearance at WPFB in Middletown, Ohio. At his father’s insistence, Bobby sang “Ruby” for the first time, and 50 telegrams were received by the station asking them to have Bobby sing it again. They did, he did, and as they say, “The rest is history!” That song became a signature for The Osborne Brothers throughout their career.

Ruby was coal mining folk song and young Osborne had probably heard it first while in the cradle. The Osborne brothers were born in the heart of the Eastern Kentucky coal fields. They were born in Roark, Kentucky but moved to Hyden after the family home burnt down. One supposes that Bobby wasn't inclined to push his luck as his debuting song became his signature song for the next 20 years until he and his brother recorded Rocky Top on Christmas Day of 1967.

The end of World War II saw a large out migration from Appalachia as thousands of backwoods country boys found opportunity abounding in the Northern industrial cities. They met the world on their own terms, sequestering themselves in hill billy ghettos in cities such Detroit, Chicago, Cincinnati, and Dayton. They kept their churches and their music. More aptly they gave the world their music. The Osborne family moved to Dayton and the brothers went about the business of selling Bluegrass to the world. The Osborne Brothers became the first bluegrass group to perform on a college campus at Antioch College in 1960 and the first bluegrass group to perform at the White House in 1973. They joined the Grand Ole Opry in 1964.
Other than having a demanding father who was apparently their biggest fan, in Bobby the most recognized tenor voice in bluegrass that could hold a note all afternoon, in Sonny a banjo player good enough to play for Bill Monroe at 14, the creativity to conjure up their unique "stacked harmony" and the show businesses acumen to hitch onto the folk music trend the brothers didn't have a lot going for them after Bobby took time out to fight in Korea and win a purple heart. If Sonny was musician enough to play for Bill Monroe at 14 Bobby was creative enough at 17 to write Pain in my Heart that was later recorded by the legendary Flatt and Scruggs.

Before he had a career Bobby tried to emulate Ernest Tubbs, even playing an electric guitar. That all changed when his dad took the family to hear Bill Monroe when he played Dayton. He continued with his guitar until he teamed up with Jimmy Martin who wanted to play guitar. He then made himself one of the best mandolin players in the business. Savor this account of Sonny's banjo background from the Basement Rug.

He began by convincing his dad to buy him a $100 Kay five string banjo which they ordered through the school music department. Before the banjo arrived, Sonny remembers sitting in class at school, trying to figure out a Ralph Stanley break on “We’ll Be Sweethearts in Heaven”. Sonny felt he had the right hand figured out for that song, and also “Cripple Creek”. A few weeks later when the banjo finally arrived, to the amazement of his Dad, music teacher and himself, Sonny was immediately able to play it. He practiced at least five, and sometimes as much as 15 hours a day, out on the back porch swing. Often he’d still be up at 4:45 a.m., at which time he would hurry off to bed and pretend to be asleep before his father awoke for work at 5:00 a.m. and checked on him. Sonny says his father never was the wiser.

Then he hired out to Bill Monroe

Bobby was in Korea at this time, in the United States Marine Corp. When school was out in June of 1952, Jimmy Martin and Sonny went to Bean Blossom, Indiana to see Bill Monroe. Bill hired Jimmy, and with Jimmy’s insistence, also hired 14 year old Sonny. A week later they were off to Nashville. On Sonny’s first Grand Ole Opry appearance with the Bluegrass Boys, he performed “Rawhide”. It was during this time period when Sonny recorded nine tunes with Monroe. This was quite an experience for a 14 year old.

Key to the brothers' success was their stacked harmony vocal style. This technique enabled Bobby to sing lead at the top of his range during verses, and stay on the high lead in the chorus. Sonny sang baritone and usually another vocalist sang low tenor. That produced very smooth harmonies, but without the lonesome quality. Their music never sounded forlorn. The Osbornes offended some in the "we don't plug in"  community when they used electric instruments and even drums but they had livings to earn. It's not as if some fiddlers didn't admit that they borrowed their own patented technique from from a clarinet player or that Monroe himself loved to go clubbing in New Orleans to savor jazz and dixie land. Sonny retired in 2005 following a stroke but Bobby continues to perform.
Here is an example of that stacked harmony at its very best.

So where the Osborne Bothers, Bill Monroe, Lester Flatt and Earl Scruggs able to sell their music to the world ? Could be the answer is yes. Ask someone in the Korean audience if he likes Japanese bluegrass.