What was the primary reason that the Yardbirds had to replace Eric Clapton in 1965?

Answer 1

The quintet went from there to cut several singles, including "I Wish You Would", but it was their third single, "For Your Love", a Graham Gouldman composition that was anything but the blues, which put the band to their highest chart position yet in England—and gave them their first major hit in the United States when it was released Stateside in 1965. The group's move into pop outraged lead guitarist Eric Clapton, at the time a no-holds-barred blues purist, who had already doubted the ability of "nice college kids" like bassist Paul Samwell-Smith to play the "real blues". Clapton left the group in protest, subsequently joining John Mayall's Blues Breakers.

Answer 2

He was fed up with their limitations.....and besides.....John Mayall was beckoning......much more up Eric's street.