At least a hundred covers of Yesterday? The number of covers passed 2000 several years ago and is probably at 3000 by now. I can’t find an exact number with a quick Google, although I did see over 2500 on one site.
Although the other posters got the basics, a few more reasons apply.
Throughout the history of popular music, much of the truly innovative work would appear first from small, even tiny, record labels who were willing to take chances that the majors weren’t. The problem would always be that these small companies didn’t have the distribution strength to get their records out everywhere. Although white artists in the 50s routinely made “safe” versions of songs by black artists, many of the covers were almost duplicates of the originals. It was a way of getting good music out to the public who would never have heard it otherwise.
Additionally, songs as songs were more important then. If a good song came along, you recorded it. And the public didn’t care much who the singer was. There were times in the 50s when five versions of the same song hit the Top 40.
That changed somewhat in the 60s, when the singer/songwriter and songwriting band came along. But not completely. Everybody who recognized a good song covered the good songwriters, and covers of Bob Dylan, Joni Mitchell, Laura Nyro, and the other greats appeared on everybody else’s albums, especially those people who happened to be sleeping with them at the time. And in the 1960s that apparently was everybody.
Then there was Motown. Motown was a throwback to the old ways, with their writer/producers who churned out songs and gave them to their individual stable of artists - and stole them from the other writers’ stables. You’ll find that many Motown hits had earlier versions by other Motown groups, but with inferior productions that didn’t quite work. The most famous is “I Heard It Through the Grapevine.”
Written by Norman Whitfield and Barrett Strong, it was first recorded by Smokey Robinson and the Miracles but not released until put onto a later album. Then the Isley Brothers recorded it, but Motown never released that version either. Then Marvin Gaye recorded his superhit version - and they buried that as well.
It wasn’t until Gladys Knight and the Pips took the song to number two in late 1967 in yet another totally different version that anybody at Motown realized that the song might have some real value. After letting a year go by, they put out Gaye’s version and had one of their biggest hits ever.
And they didn’t stop there. Whitfield had two other of his groups - the Temptations and the Undisputed Truth - also record the song.
There really aren’t any production factories like Motown around any more.
A related reason for covers in the 60s was the difficulties of getting records distributed across the Atlantic. All the British Invasion groups from the Beatles on down loved the odd little 45s that their obsessive selves managed to get their hands on but which never got formal air play in the U.K. And so a million songs from “Chains” to “Diddy-Wah-Diddy” are covers of obscure U.S. songs and b-sides.
Same thing in reverse. American producers would hear a British song and create a cover band to do it in America before it could get distributed, the most famous being the Shadows of Knight and their note-for-note cover of Them’s “Gloria.”
That’s mostly not necessary any longer either.
Today bands cover songs just because they feel like it. And it doesn’t always take years to do so either. When you have the time and patience, go carefully through The Covers Project for more covers than you ever realized existed in this universe. And remember that it is a very partial, selective, and half-assed job of amassing them.