Well, because they have been terminated by Spotify, of course. I mean, it is pretty easy to imagine that your preferences are "free Spotify with blocked ads is better than Spotify / Apple Music / etc. for 10+$", but that after the "free with blocked ads" option disappears you will (grudgingly) end up paying.
And the right move by Spotify would be to support this transition in a way that does not drive this business to their competitors. If you end up moving these people to choose between "ok, now I have to contact customer support to get my account unblocked so that I can then start entering my credit card details" versus "ok, I can just use this big 'Start Free Trial' button here on Apple Music", I don't think that is a very difficult choice.
Also, while Pi-Hole might be quite niche, the point I was trying to make was more that many people use adblockers (about 20% in Europe[1], far bigger than a rounding error), but many of those probably don't do so specifically for Spotify, this is just a side-effect of running Spotify in the same browser. So they might be amenable to a warning that they should disable their adblockers on Spotify, while just terminating their accounts will just drive these users to competitors.
[1] https://www.quora.com/What-is-the-percentage-of-Internet-use...
IME, cheap people are cheap. Nagging them doesn't change their minds.
happened to use Spotify for a while on a system with adblocking enabled
drive a lot of potential customers
If they're not paying and blocking ads, they're not customers. And if they weren't willing to pay for Spotify, why would they suddenly be willing to pay for Apple Music?
being on a Pi-Hole network
The number of people setting up home filtering devices isn't even a rounding error to a service the size of Spotify. All of those people could disappear and they wouldn't even be noticed.
Ditto for the number of people who might just happen to end up on an ad-restricted network at a friend's house.