Because "email marketing" means "spammers". They were buying new blocks of IP addresses to try to evade blacklists. They are the scum of the internet, dumping their pollution far and wide because nobody is stopping them.
*to be clear, not all email marketers are spammers. But you can be damn sure anyone buying that many IP blocks is. There's literally no legitimate reason for them to need that many IP addresses.
My guess is that their subnets would be periodically marked as spammers in various black lists, so they would need new subnets to continue "email marketing".
At the same time, where I work, at least with ipv4, we like to segment product by ip block. This way if one gets a bad reputation it can't adversely affect the others. This hasn't been an issue in practice, but its just an extra layer of protection we like to have.
I'm not sure if there's the same concern with ipv6 or not.
*to be clear, not all email marketers are spammers. But you can be damn sure anyone buying that many IP blocks is. There's literally no legitimate reason for them to need that many IP addresses.