Ultimately, it had to do with the tumor that David Boreanaz's Booth was contending with (more specifically, a symptom of that tumor): "We did the little crossover with Stewie from 'Family Guy.' ...

I have no problem with the cult audience, and I would totally get back into the genre, but I'm not a big reunion guy ... David Boreanaz's SEAL Team was a proud member of the CBS family.