Because that’s the way everyone expects pop and rock musicians to sound. British pop singers have been imitating American pronunciations since Cliff Richard, the Beatles, and the Rolling Stones began recording in the 1960s.* These musicians were largely influenced by the African-American Vernacular English of black American blues and rock and roll singers like Chuck Berry, but their faux-American dialects usually comprised aspects of several American dialects. Imitating an American accent involved both the adoption of American vowel sounds and rhoticity: the pronunciation of r’s wherever they appear in a word. (Nonrhoticity, by contrast, is the habit of dropping r’s at the end of a syllable, as most dialects of England do.) Sometimes Brits attempting to sing in an American style went overboard with the r’s, as did Paul McCartney in his cover of “Till There Was You,” pronouncing saw more like sawr.
The rest of the article focuses on linguist Peter Trudgill who tracked rhoticity in British music and discovered that the Beatles’ pronunciation of r’s decreased over the 1960s decade.