I dont know if I would say there is a full on resurgence. The numbers are still relatively tiny compared to the 90's. Perhaps re-awakening is a better description, but that's just semantics.
I agree with the previous suggestions as to what is keeping pinball going now. Also, if people want to play, they typically don't have the option of readily available machines in the wild.
Further, there has been increased availability of repro parts, especially for 90's era machines, making them fully restorable. That with the readily available info on collecting/fixing/playing online.
Also, people who have access to the funds are willing to buy machines knowing they hold their value and in some cases might appreciate.
Since the industry has recognized that the consumer market represents a significant chunk of their clientele (finally), they are ramped/ramping up to meet demand. Knowing that resale and collectibility are essential to keeping the consumer market going, LE type machines in limited volume is intended to facilitate this. Boutique pins is a byproduct of a lower volume higher resale/collectibility market.
Incidentally, the age of the average pinball collector who is established financially enough to support the new machine market is such that they remember the rise of the Web in the mid 90's, have gotten over onlline gaming and entertainment from a novelty perspective and want to go back to something with real physics (and tech). In fact I believe the internet is partially to blame for killing pinball in the day.
A line of flashing complex looking pinballs is impressive to show off! It sets you apart from most people, who's interests might not be as fun to share.
That is my take on things.