I think the larger reason most pinball manufacturers go for older themes is simply because they’re a safe bet in many respects.
As far as I’ve seen, most people who buy new pinball machines for home use are people in their 30s, 40s, and 50s. I always figured this is part of why nearly all music-themed games are based on rock groups whose heydays were no later than the 1980s or early 1990s – it appeals to that particular demographic. The other side of the coin is also important to consider here: operators obviously want something that the general public would recognize so that they can make more money off of their games, and regular Joes are more likely to go for a band they recognize and like over a newer one they may have never heard of. (Likewise, it’s difficult to tell which popular acts of today will end up having lasting popularity – financially speaking, it’s a no-brainer to go with a tried and true group that still attracts fans decades after the fact.)
Moreover, the fact that the market’s shrunken significantly from its '90s peak makes it more difficult to justify taking a risk and making a game based on a newer, unproven license. The days of Congo and Johnny Mnemonic are long gone – put bluntly, brand recognition is important for a game with any sort of wider release. (Of course, machines with more limited runs have some more leeway here: Rob Zombie and Primus are good examples, since their followings aren’t large enough to warrant something like a Stern cornerstone title.)
To me, it seems like a quiet but vicious cycle: the market’s just small enough that a major title that’s not recognizable to a significant portion of people is a risky proposition, but continuing to rely on familiar brands from decades past constrains pinball’s audience. I think Stern hit upon a decent compromise with Stranger Things (a series replete with '80s nostalgia) and The Mandalorian (a modern installment in an immediately recognizable franchise), and Spooky managed to get an immediate hit with Rick & Morty, but I’m not sure the state of affairs will change on a larger scale in the near future.
It’s interesting to see people on Pinside debate about what would make a good “modern” theme that would appeal to younger audiences today. Pokemon gets brought up a lot, and while it stretches the definition of “modern”, I’d say it’d have a lot of potential and has quite a bit of cross-generational appeal. Minecraft and Fortnite could also work - they’re undeniably popular if nothing else. I think something that constrains a lot of other popular themes is that they haven’t quite hit the mainstream in the same fashion all three of these have. (That said, My Hero Academia might have enough recognition to break that mold.)
(Side note: part of me wants to see what Spooky would do with Hazbin Hotel 
)