There were thousands of song performances that never got ANY airplay or sales, that I think were much, much better than most of the cuts that charted and were hits. There were probably several causes that contributed to that happening. One of them was lack of business acumen of the group or single artist's manager and the label owner and R&D man of the labels that released the records, in addition to the lack of funds by the given label needed for proper distribution, numbers of records pressed, and payola to get it played, and money to get into the music business with enough credibility to have access to the people who could market the record and get it heard by the right people.
Also, first, the "British Invasion" changed the situation in the Pop record market, and the resultant changes by the US major labels' market strategies also started a slow drift away from R&B and Soul in the Pop market starting in 1964, and continuing throughout the remainder of the '60s and into the '70s.
The Major US labels (Columbia, RCA, Warner Brothers, Decca, Capitol, 20th Century Fox, and to a lesser extent, ABC/Paramount, didn't really know how to market their R&B and Soul product.
Motown, alone, had literally thousands unreleased cuts that were better than most charted Pop cuts. There was just no room for those to make the charts. There were so very many talented singers who could have been stars. There just was no room for all of them. Most of them had no chance from the start, because they were never seen, nor heard, by the right people.