So why can't the same happen for Castiel from Supernatural?
Judging from the comments that have been made, not just from today via the recent interviews but in general, there is a massive split between the fandom about whether the show should have characters like Castiel staying or that it should always be about the brothers and no one else. The show has always been neglectful with their minor characters, many of which had much potential and they just never fully developed them or took advantage of such amazing opportunities. Having a spin-off would bring those opportunities to life that the show wouldn't even consider, let alone allow them to have. Which doesn't make sense since SPN isn't even a huge ensemble show anyway so this shouldn't be a common occurrence of abandoning/killing off characters just for the because factor, but I digress.
Castiel is a fan favorite, the majority of the fandom fell in love with him the moment he was introduced and have been rallying, campaigning and showing love and support for him ever since. You'd think that they would take the hint that perhaps people want more Castiel elsewhere instead of on the show where his storylines often get pushed off-screen. And since this is all about the money nowadays, this would double their chances at more viewership and the ratings, which is precisely what the network wants.
I mean come on, if the Ghostfacers can have their own webseries I'm sure Castiel can most definitely have his own spin-off. I'm just saying.