Probably because with IPv6 privacy is built-in somewhat into the protocol, eg you can have a different IP really easy. For example, I can see my desktop right now has 7 different addresses.

Now, you could truncate this to eg a /64 or /56 range to identify users, but each ISP has different rules. Mine gives a /56, but I also hear many give only a /64 or less.

As such, it basically means that you can’t really rely easily on IP addresses anymore for spam detection, rate limiting, etc.

Note that I’m not an expert on spam filtering, but I do have quite some networking experience and QoS, and ran into these issues a lot.