I think there are several factors. For the record, I didn't want Tubby to go. At all.
1. They haven't had the same level of success. Several posters above me have expounded upon it.
2. It's fun to ALWAYS be in the conversation. It's not entirely logical, but it truly is.
3. A decent portion of UK fans are insane. I mean, there is small percentage that represents themselves rather loudly that would like to see Cal go.
4. I think it's a rather small factor here but Tubby was black and Cal isn't. Racism exists, to say it doesn't is foolish. However, if Tubby had the same winning % and got to the FF the same amount I think he would've stayed longer. Winning overcomes a lot.