Because IPv6 addresses are free and IPv4 is expensive. Same reason why Google won't let you sign up without SMS verification. If you're caught spamming or breaking TOS you've effectively burned that v4 address or phone number.
v6 is more difficult, by design. The lower half of the address is deliberately not subnettable and it is the explicit design intent that machines on a v6 network can just make up new addresses within a /64 as they please. So you have to burn subnets. Except there isn't really a standard for how subnets are issued: most ISPs hand out /48s, Comcast insists on /64s for residential use, etc. In the IPv4 world you could ban one IP at a time, and only move on to banning entire AS allocations if you needed to. On IPv6, banning a /64 is a lot less impactful, so you have to start with the most drastic and customer-hostile option.
Comcast hands out a /60 for prefix delegation if you ask for it (i.e. software asks for it, no customer service interaction required). In fact Comcast allows you to ask for as many /60's as you want (caveat, there may be a limit, but at one point I made a config mistake that led to asking for 32 /60's and I got all of them, so I am not aware of a limit).
v6 is more difficult, by design. The lower half of the address is deliberately not subnettable and it is the explicit design intent that machines on a v6 network can just make up new addresses within a /64 as they please. So you have to burn subnets. Except there isn't really a standard for how subnets are issued: most ISPs hand out /48s, Comcast insists on /64s for residential use, etc. In the IPv4 world you could ban one IP at a time, and only move on to banning entire AS allocations if you needed to. On IPv6, banning a /64 is a lot less impactful, so you have to start with the most drastic and customer-hostile option.