Because Russia could easily start a conventional war and there is no way NATO would escalate that to nuclear without Russia using nuclear weapons first.
NATO has never publicly committed to a NFU policy because that would weaken their hand on the political stage but in practice they seem to be following that line. It's a smart way to position themselves, and other nuclear capable entities have done the same.
NATO no longer faces this threat, but the policy remains.
Of course, the problem is that Putin has a very expansive idea of "what threatens Russia"...