I think it goes back to Tulisa being Cheryl's replacement on X Factor, and with them looking quite similar and having a similar background the press and the public caught onto it at first and just accepted it.
Fast forward a year and Tulisa was exposed as having a millionaire father (and so didn't have the same background people thought she had), her music was truly awful, failing to chart high despite being promoted on X Factor, the sex tape leak, and now the cocaine thing. She has become a bit of a laughing stock which never happened to Cheryl. Whilst Cheryl has a lot of fans and at one point seemed untouchable, she still has her detractors but doesn't seem to generate anywhere near the same level of hate as Tulisa does. And a lot of anti-Cheryl comments were due to overexposure.
There was the expectation of her being the new Cheryl, and she probably could have been, but ultimately poor management and has insistence to be "real" has ruined her and her career.