FO is not that big of a IP compare to SW or COD .
Fallout as a franchise has never been on my radar. Doesn't fit in with my gaming tastes at all. I was really taken aback during E3 at just how big of a following the game has. I just though that was this vaguely popular game, but it's a multi million selling phenomenon with a dedicated fanbase. Fallout as an IP is massive.
The Fallout 4 reveal trailer on Youtube has been viewed 15.6m times. If memory serves, it hit 10m within a couple of weeks of being released.
Fallout 3 launch in Oct 2008 and by the end of the year had sold 4.7m copies. Fallout NV sold 5m in a month, and both saw a huge uptick in sales after FO4 showing at E3.
Based on the pre-orders for it, GAME (the UK's biggest retailer) is estimating that FO4 will outsell Skyrim (which has raked in over 20m sales).