Given the length of the suspension, I wonder if NBC did some more detailed fact checking and found that some other Brian Williams stories didn't add up as well. The long, immediate suspension means that NBC can do that research without the continuing issue of B-W being the face and leader of NBC news. The possibility of other "mis-remembered" or embellished incidents and events is too much for NBC to handle with Brian Williams remaining as news anchor.
This is the downside of having a news anchor go on various late night talk shows (that deal mostly in comedy and entertainment) as a guest and "star." His stardom was based on his news reporting, from an "I went there, saw things with my own eyes" vantage point. Thus his stories contained both the perspective of the reporter on the ground "in harm's way" seeing things up close and personal, and the "behind the anchor desk" reader of news collected by others, bringing the big picture perspective to the news. Those things are what made Brian Williams the number one news anchor and why his name was part of the title of the NBC nightly news broadcast. With the questions about his on the ground experiences, his whole career and presence becomes tainted until the details become more clear. Thus the lengthy suspension.