So there's been a theory going around that Ultra Beasts are based on pests since the jellyfish, mosquito, and cockroach are an annoyance on Hawaii. This theory hinges on UB-02 Beauty being based on a cockroach but perhaps it's based on the copepod.
UB-02 Expansion seems to be based on the mosquito. The mosquito is an foreign invasive species. There's a type of copepod called the Macrocyclops albidus that are used as a form biological control (biocontrol) to keep the mosquito in check.
Here's a comparison of the two:
If this is true, then it would make sense that they are version exclusives, as they are counterparts of each other. They also wouldn't be the only version exclusives based pest vs biocontrol as Gumshoos and Alolan Raticate are version-exclusive totem pokemon. Perhaps there will be other version exclusives/differences based on this concept.
Other interesting facts I came across regarding invasive species and biocontrol:
*UB-01: Jellyfish aren't a foreign invasive species but they do migrate and invade Hawaiian shores regularly. What's interesting is that there's a pattern to it. They are typically seen in Hawaii about 9-12 days after every full moon.
*Cutiefly: There have been attempts in the past to use bee flies as biocontrol of grasshoppers. Its evolution seems to be based on some kind of moth or butterfly. Hawaii recently introduced the Madagascan fireweed moth to control an invasive plant species, the fireweed.