Several reasons:
FreeBSD is a fully fledged OS with GUI and everything and multiple gigs in size. FreeNAS is only 100MB and runs from a flash-drive. On boot the 100MB image is extracted to memory and runs directly from memory.
FreeBSD doesn't have any type of interface for the various storage related tasks. FreeNAS provides a GUI where you can do almost everything storage related directly from the WUI.
FreeBSD contains many, many modules you won't use in a NAS. FreeNAS only loads the essential kernel modules, even basic kernel modules are excluded when they aren't required.
And to clarify:
FreeNAS 7.xx is based on FreeBSD 7.2 and 7.3
FreeNAS 8.xx is based on NetBSD 8.xxxx
FreeNAS 8 is a COMPLETE rewrite of the entire FreeNAS system but it is still in testing (RC1 was released a couple of days ago). For now I'd stick with FreeNAS v7.xxx until v8 is released and maybe even wait for a second release.
Hardware wise, I would recommend this:
Motherboard: Intel Burrage Flat P67 (Extreme Series), P67BG
CPU: Intel Core i5 2100T (low power CPU)
RAM: 4GB minimum, but 8GB recommended
PSU: Corsair CX430 430w
Case: Cooler Master Centurion 590
Flash-Drive: At least 1x2GB or larger flash-drive for the OS
Hard-drives: Any hard-drive but 2GB or larger recommended. At least 3 of them so you can use RAIDz.
That setup is specially selected for several reason:
1) It uses an Intel Network card (very important)
2) It uses Intel SATA controller (well supported)
3) It uses a low power CPU (a NAS doesn't need a too powerful CPU and it runs 24/7 so it should be low power)
4) The case has 9 5.25" bays (You can install 9 3.5" hard-drives, although you need 5.25" -> 3.5" converters)
5) The case allows 3x 120mm fans to be installed in the front for ventilation of the hard-drives
6) The hard-drives have sufficient space between them to allow for good ventilation
7) You need at LEAST 4GB of RAM for RAIDz to work well