The beach boys were so much more commercially successful in the late 60s in the UK because americans had pretty much given up on rockNroll.... hippie protest music and psychadelia were what was being pushed and marketed in america at that time
A quick look at, say, the Billboard charts from 1968 tells a different story: Dock of the Bay, This Guy's in Love with You, Mrs. Robinson, Tighten Up, Mony Mony, Cry Like a Baby, Midnight Confessions, Dance to the Music, I Wish It Would Rain, Judy in Disguise, Spooky, Simon Says, A Beautiful Morning, The Look of Love, Yummy Yummy Yummy, 123 Red Light, Jumpin' Jack Flash, Nobody But Me, I Wonder What She's Doing Tonight, Shoo-Be-Doo-Be-Doo-Da-Day, Elenore, Baby Now That I Found You, Do You Know the Way to San Jose, Mighty Quinn, I Say a Little Prayer, Suzie Q. These were all big hits in 1968 and had very little to do with either protest music or psychedelia.