At least partially because you only have another year of support for FreeBSD 10.x, whereas FreeBSD 11.x is expected to be supported until 2021. This includes security fixes.
Generally speaking, more conservative users will wait at least until the xx.1 point release before switching to that major version, but if you're on the very conservative side you may want to wait until xx.2. However, there are numerous updates even between minor point releases which you could apply if any errata are discovered if you don't mind.