Taken from Wikipedia:
“In the years following Bet's departure, ratings for Coronation Street started to become overtaken by rival soap EastEnders, with long-term viewers complaining that storylines in Weatherfield were becoming increasingly depressing. In 2002, producer Kieran Roberts was keen to help the show return to its original roots and bring back a character from the 'golden era' of the show. Julie Goodyear was offered a one year contract to reprise the character and Bet returned to Weatherfield in June 2002 for Betty's retirement party at the Rovers. Offscreen, Goodyear struggled with the increased shooting schedule of the show and, amidst unconfirmed reports of 'diva' like behaviour, quit after 17 days of filming on the advice of her doctor who diagnosed her with stress and exhaustion. Goodyear's abrupt departure meant that Bet's storylines had to be concluded off-screen. In November 2003, Bet made her last appearance to date in the show, with a storyline that reunited her with old friend Liz MacDonald in Brighton.”
en.wikipedia.org