Angel started season 1 as Buffy wenrt to season 4. Joss made Angel a darker more adlt/gothic version of Buffy and so he wanted a more adult cast. Cordelia wa a more mature role than say Willow, though the way Cordelias character went from the dippy fashion fanatic to the efficient secretary was amazing but also suited the new career change! Wesley was the required English type and Giles was aleady well installed in Sunnydale. Don't forget Doyle though, if Glen Quinn hadn't died i think he would have made a strong return.,