The quantity-above-quality model in the spam industry probably lead to low-quality or vague list criteria. Broad demographic groups and domains. A lot of third party data of questionable legitimacy. If you've been running the list a long time, some "email address foo@bar.com clicked on campaigns A and B" data. You might be able to say on a large scale "List A is likely to convert better than B", but it still comes down to spray-and-pray at the individual address level.
If you went to a spammer with a few "sucker lists" of 100k emails each, and told him "you can only send 1k per day", will he have the data to triage his list to get a decent return on it anymore?
This would also snowball over time. Without being able to do high-volume campaigns initially, they'd have a harder time building up the knowledge they can use to manage their lists, and the quality would decline over time.