Thread by @RavelMonte

Why do Digimon evolve the way they do?
Instead of a linear path, it's well known Digimon evolve in ways that it's easier to say what a Digimon can't evolve into than what they can. But why? Let's look at the development history and see many things that affected Digimon.

In the early concepts, when Digimon still was Capsule Zaurus, the designs were much more simplified and focused on creating elemental monsters. A much more linear and simplified approach could be seen here but for trademark and copyright reasons, the concept changed completely.

When it came to making the actual Digimon, the first ones made were Agumon and Tyrannomon. Since they were two dinosaurs, it's basically a more "cool looking" version of what was thought about Capsule Zaurus.

But Kenji Watanabe, the artist, was inspired by American comic books and he wanted something that could really represent that, so he made Devimon. It was very different from Agumon and Tyrannomon and opened the doors that Digimon could be anything, not just dinosaurs.

There were no actual plans (Other than Horimura's hopeful thinking) of making Digimon anything more than a single toy, so the staff decided to just make as many distinct designs as they could, so the first Digimon toy covers everything from the sea to the ground to the skies.

Still, there were a few evolutions that felt more connected. Agumon into Greymon and Metal Greymon, Betamon into Seadramon. Making more logical evolutions was on the table, but that didn't happen because of the system the toy was being made.

Digimon was being made using the same system as Tamagotchi, which meant that the overall structure was also more or less limited to what Tamagotchi could do. The technology had improved, but not by much. So a lot of Digimon was defined by the limitations of technology.

For example, the Digimon lifespan is limited by design because the toy didn't have a memory that could survive being without a working battery, so they set the lifespan of a Digimon to just fit under two or three generations before the battery ran out and everything was reset.

In the original program, there were only 13 planned spaces for characters, they got to use 14 because they made the 14th with a very small size, Mamemon. If they had more space to cover all the points they wanted, there could be more Digimon to allow evolution to make sense.

When they got to make new versions, the system had already been established by the 1st toy so they just followed that. They tried to have each version to be more self-consistent, but they couldn't make meaningful changes by this time gameplay-wise.

What they tried to do was to make an in-universe consistency with evolution, so even though in the gameplay you had crazy evolutions, in the background there were connected Digimon that were made to make sense together, starting to plan out various "orthodox evolutions".

With the next generation of Digimon, Digimon Pendulum, they had it all already planned out with a lot of the lore made from before, so with the structure of the new toy they focused on having each version be consistent and created as many Orthodox evolutions as possible.

Now with a proper framework and planning, they could make Digimon with direct connections, but what was done was done and unrelated Digimon could still evolve into each other, which was already established as the Digimon evolution system.

Of course, for those who like lore, there's a lot of background to explain why Digimon evolve the way they do and why in essence any Digimon can evolve into anything as it makes sense for them. Still, I'm sure I already wrote about that.

In the end from those who worked on Digimon back then, most seem to agree that the lack of limitations is what really makes Digimon shine. So throw away any concepts that "Digimon has to be X", they are Digimon, so they can be anything.