Well, that's kind of a weird thing to say. pfSense and FreeNAS are not "FreeBSD releases". They are products built on top of FreeBSD, designed to make FreeBSD easier to use. Since users expect that the management layer offered by these appliances will work correctly, and since that's all very dependent on changes that may have happened between releases, including things like new features, changed drivers, etc., there's a certain amount of qualification and validation that goes on to make sure it works. That takes time to write code to adapt to the new features, more time to test those changes, etc.
You may note that FreeNAS tracks FreeBSD 9, not FreeBSD 10 or FreeBSD 11, because that is where the most significant active development is going on, and therefore also where the greatest risk lies. For an appliance, there's no need to have every latest and newest feature if it might mean compromising the stability of the platform. Instead, a more conservative approach means that a new disruptive (and irrelevant) feature in -CURRENT isn't going to be an issue to your storage appliance's stability.
Also, in the IT world, it is quite common for a company like Cisco to release a new version of software and for companies not to deploy it on day one, because it is invariably better to let someone else discover if there are problems with the new release. Because sometimes there are. So when FreeBSD 9.4 comes out, it might be a little while before a FreeNAS release comes out based on that.
Further, iX has based FreeNAS off of TrueOS, which is based off FreeBSD. This puts the brakes on change propagation even more, and allows them to pick and choose certain features. It is a lot of work to go that route, but it pays off in that we've had a very stable FreeBSD variant acting as the basis of FreeNAS.
It's a more professional model than what might go on elsewhere. It isn't perfect but it has been cranking out a great stable server OS for twenty years.