I have a theory.
Keiji Kinebuchi made the artwork for Basic Energy, and was credited for it, back with Base Set. He stopped being credited on Basic Energy cards starting with Energy cards in the e-Card era, which is coincidentally when he stopped making new artwork for the TCG altogether. While other art that got reused could easily be credited and moved on, Kinebuchi's Energy symbols are the ones still used on every Pokémon card today – its type, its attack costs, its Weakness, Resistance, and so on. Whether he quit or was fired or a contract wasn't renewed or what, we don't know, but obviously the arrangement ended. Rather than continue to credit Kinebuchi on every single Energy card in perpetuity, it seems like they might've negotiated a nice deal with him - maybe a big one-off payment as part of the exit package, in exchange for not having to credit him on a Basic Energy ever again.
Special Energy were something of a different thing, since even during Kinebuchi's time he didn't have a monopoly on making art for those – Takumi Akabane, Milky Isobe, probably others had done cards like Rainbow Energy and Miracle Energy. So Special Energy was probably considered separate, and credits continued ...up until the end of Generation 6. The last Special Energy card with an artist credit on it was Keiji Kinebuchi for XY Evolutions' Double Colorless Energy.
Feel free to wildly speculate on whether that's a coincidence or not.
Regenerative Energy
