A third possible reason

I've already mentioned two possible reasons why the Bushies would avoid the already lenient FISA rules for wiretaps: They were bugging people they shouldn't be, and they just want to establish the point that they can do whatever they want. Jonathan at Past Peak offers a third possibility:

So why didn't they use FISA? One possibility is that they were monitoring people they shouldn't have been and they didn't want anyone to know, not even a special national security judge operating in secret.

Another possibility, though, one that strikes me as more plausible, is that they were monitoring so many people, so many calls, that they didn't want the numbers to show up in FISA statistics. I.e., what they've done is put in place a broad-based, illegal, automatic call-scanning mechanism. FISA stats would have made that clear, so they couldn't go there.

In the context of a broad-based call-scanning operation, their complaint about FISA being "outdated" starts to make sense. FISA protects civil liberties by requiring that monitoring be targeted and specific. Under FISA, the NSA can't legally go fishing by scanning bazillions of calls. With good reason.