Wotc's been trying to change Magic's public perception from being that weird card game that people keep confusing with Pokémon and D&D to the full-fledged cultural juggernaut that it really deserves to be. They sacrificed a lot of the humor and whimsy that made old school magic so fun to try and give the game a more serious image.
A cultural juggernaut, as Marvel, Disney, and others have proven, is bolstered by the strategic fusion of corporate universes and associations. When you connect a franchise with another, you suddenly meld the interest of millions of people, creating a product that pushes the boundaries of each franchise's target audience. This expands the realm of each franchise's influence massively. It is an age-old trick, and now Hasbro's starting to learn it (for example, by putting "Sword of Dungeons and Dragons" in the Un-set you praise so highly). Crossovers are, in fact, part of the recipe for cultural domination.
Even the silver-bordered set released in this era was treated seriously, despite the fact that it was partially a joke. Unstable was basically just a set playing with design space black-border magic couldn't use with an element of humor.
What do you mean by "treated seriously", and how does it conflict with jokes? One powerful way that people gain the respect of others is through good-natured self mockery. By making fun of yourself and laughing with the crowd, you show authenticity, confidence, and likability. The same strategies can be employed by companies. Instead of displaying insecurity in regards to Magic's ties to sillier and cheesier topics, Unstable succeeded by turning it into an enjoyable parody that conveyed self-assurance.
The plan was starting to work- there are more magic players now than ever before.
I could make up any number of explanations for this trend without any of them resting on solid evidence. However, for the fun of it, let me throw out some other, potential reasons for this:
-more widespread acceptance of 'gaming' in general, primarily due to its tie to internet culture, which, thanks to the meteoric rise of the internet, has bled into overall global culture
-population growth, or more crucially, growth in the number of people in the world with enough disposable income to spend it on cardboard
-people who grew up with magic are now in their 30s-50s, at the height of their income and purchasing power, and their nostalgia is being heavily cashed in by WOTC
-it is easier than ever to learn the game than ever with Arena, Youtube guides, etc
-as more people play the game, they recruit more friends to play, who may recruit others, and so on
-enfranchised players often are involved in Magic for a very long time. Also, most enfranchised players are under 65 (because Magic started in the 90s and primarily targeted young people), so few of them have died yet. Even without much new blood, it might be possible for the total number of players to keep going up, despite the
growth slowing
It's really frustrating hearing that parts of Magic that you really enjoy aren't ok for the game because they're either not serious enough or too complex, only to be slapped in the face with cards that have the words "Everypony" and "Coat, Mane, and Outfit" on them.
I mean, yeah. "It's not okay for Magic" is code for "it's not profitable". This OC was deemed profitable, and that's why WOTC made it happen. However, complexity at high levels has been deemed unprofitable by WOTC, and as a result we have not seen Time Spiral levels since that block (though Modern Horizons certainly represents a step closer).
It's already hard enough trying to be included with non-gamers openly playing Magic these days. Did they really have to make us look like Bronies too?
Most people not involved with Magic have not followed the creation of this niche product. If you don't play with said pony cards, I think you will be fine...
Imagine for a second if they had used this product slot to- say- reprint 3 Planeswalkers with new art. They would have given more to Charity and made a product that makes Magic look good. Instead, we get cards featuring characters from a line of toys for toddlers.
Magic's mythos is being commercialized and expanded. It started with the creation of the Multiverse. Then we started to get planes that were mock-ups of pop culture settings like Ancient Greece. The envelope was further pushed by Vehicles and other technologically-associated props. Magic started to become more pluralistic and more careful about its image. We got more and more products that stretch the definition of Magic, from a digital platform structured like a phone app to this most recent crossover. At this point we have either a watered-down or an expanded definition of Magic, depending on your perspective. This, ironically, is the ultimate mark of franchise success, not the antithesis of it.