NSL are used to compel a party to DO something to assist with, so called, matters of national security, under threat of physical force. If the letter isn't received and a person can't be physically detained then it can't compel anything, other than hiding from its possible existence
To make things more difficult, the very contents of the letter itself are considered a national secret. Typical methods for serving papers would needlessly jeopardize the confidentiality of such papers. So one would presume that these methods simply aren't in the cards.
