What I think changed to make spam deliveries not cost-free
As I covered in my entry on why stupid spamming is wasteful, I used to think that spam deliveries were basically free (and so spammers shotgunned everything because, well, why not) and now I feel otherwise. This is not just a shift of my view; I actually feel that the situation itself changed. Which raises the obvious question of what changed to do this.
My tentative answer is that spamming became commercialized, and specifically that it became a sophisticated business. As it did so, we saw it increasingly segment into subfields with specialists and services as people realized both that you could make money selling the specialized services and that it made more sense to buy the services than do the work yourself (or alternatively, the existence of buyable services drew people into spamming who previously would not have done so). In particular, one thing that happened is that people began to rent out and sell spam sending capacity in various forms; as the spam business became sophisticated, people could buy and sell so much time on so many compromised proxies or so many delivery attempts or the like. This put a value on sending capacity, even if it was your own organically developed sending capacity (since you could always make money by renting it out to other people instead of trying to send out your own spam).
I also think that sending may have gotten more harder and expensive (in terms of time and lost opportunities). Back in the early parts of the 00s, things were in a sense really bad; there were oceans of open proxies (and before them oceans of open relays), ISPs generally didn't care, anti-spam precautions were relatively undeveloped (even at large providers), and so on. Since then many things have shifted quite far. The open proxy problem has gotten much better on many fronts (ISP cooperation, effective DNS blocklists, etc), anti-spam precautions have gotten more sophisticated in ways that hinder rapid sending, and so on.
(One inobvious but important shift is that many mailers will now drop your SMTP connection if you try to do unauthorized pipelining. Back at the height of the open proxy era spam senders simply blasted an entire SMTP conversation at you in one go, ignoring return codes and speeding up their lives. Now that doesn't really work (and spammers have by and large stopped trying to do it as a result).)