Mostly because of the number of machines that I have to fix up and the fact that updating each of them involves a fair bit of time. My estimation is I'm looking at at least a week's worth of work, during which my network isn't fully functioning.

It's also complicated by the number of devices I have that aren't possible to make work with IPv6 at all, which means I have to maintain some IPv4 segment and deal with making the two work together in a seamless way.

I'm also very concerned about security. I'm not confident that I know enough about how to secure an IPv6 network adequately, so I need to set aside a fair bit of time for study before I even start.

All in all, it's a large project with a lot of friction that wouldn't be as large if I didn't have to rethink everything.