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.
11.x may supported until 2021, but 11.1 itself will only be supported up until 11.2 is released + 3 months. So if 11.2 is released before the end of July 2018, then 10.4 which has an EOL of October 31st, 2017 would actually be supported for longer.
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.
Yes, but the point of point-releases is that they're largely about maintenance and hardware support - so moving from 10.x to 11.x might take some testing, maybe rewriting some management scripts, etc, but 11.1 to 11.2 should be easier since nothing major changes about the system. Having a year's buffer to migrate means that if there are issues, you can fix them or get them fixed before 10.x EOLs and you're stuck running an unsupported OS.
This is my number one reason for moving to FreeBSD 11.1. FreeBSD 11.0 was weird because they enabled IPSEC, but did not enable NAT_T in the GENERIC kernel.