The original FAQ was written as a supporting document for the blog post; as such, it omitted a great deal of material regarding goals and rationale. However, due to technical limitations, it became the de-facto landing spot for most readers, a role for which it was ill-suited. As a result, it was quickly swamped by unanticipated questions, unaddressed concerns, and many, many, many discussions.
Two days later, Gareth McCaughan, a long-time member of the community here and a respected moderator on our Puzzling site, wrote a much better FAQ that covered much of what had been left unaddressed in the "official" version.
The community manager team here at Stack Exchange - primarily Cesar M, Catija and JNat - took their lead from this initiative, and worked to adapt and expand on this community-driven FAQ. They've spent the past week compiling, revising and reviewing the new FAQ, collecting feedback from moderators and other concerned members of the community, and working to reconcile outstanding points of confusion or concern.
With the new FAQ released, the old one no longer serves any purpose. With just shy of 1,000 flags raised on the question, its answers, and associated comments... It had become a mess that would have been prohibitively expensive to clean up.
In such a situation, deletion is not only expedient, it is all but negligent to not delete.
To the folks suggesting a historical lock: such locks are meant for popular posts that are no longer appropriate, a way to keep culturally important things around. A FAQ that caused more harm than good does not qualify; it would continue to be a stumbling-block for readers if kept. The harm vastly outweighs the value of its short history at this point.
Also... All those flags I mentioned? We avoided handling quite a lot of them, but this would not fly if we kept the question and all answers visible - we'd be faced with the task of going through and trying to ascertain the validity of flags in a great many acrimonious conversations. I guarantee that no one involved in the many threads there would be happy with the results.