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...