I would guess that their automated spam capacity is way higher than the human capacity to respond to their newly-acquired leads. "Leads" are free in this case,

I would guess that their automated spam capacity is way higher than the human capacity to respond to their newly-acquired leads. "Leads" are free in this case, so they're spamming the initial message as much as possible because it doesn't cost them anything (and better have too many than not enough), but then their capacity to follow-up on those is much more limited so they're selective in the ones they follow-up with. Chances are that they're unsure of how well a particular spam message will perform, so they spam it as much as possible and then realize they got more than they bargained for so they don't have enough resources to attend to all the responses. Tech-wise, keeping track of leads is much more difficult. It requires essentially building an entire CRM, while an outbound-only spambot is a self-contained script with no persistent storage that just scrapes some popular post/hashtag for its targets and then sends a spam message to each one.