>>51513332They can become any other Digimon, but it's never done in a random way. Patamon (the one being the most used for describing Digimon's evolution) evolved into Angemon because he had a pure heart and was willing to fight against Devimon for example.
In every game, evolutions are determined by the stats, the education and/or the choices of the players. The only Digimon game I can think of with seemingly random evolution was ReArise and even then, they used quite common lines.
As for why they're so different, imagine Bulbasaur, Charmander and Squirtle. Give them 5 evolutions each (you don't mind Pokemon having several evolutions, I suppose), one for fighting, one for intelligence, one for happiness, one for balance stats and one punitive evolution (it's more complex than that in Digimon, but let's keep it simple). Then give everyone of them 5 evolutions. You're already at 93 Pokemons, from the 3 starters only. That's already 2/3 of Gen1. You understand it will quickly become impossible to design all of them. Now one thing you can do is reducing the number of evolutions, but Digimon focus on raising your Digimon. The Digimon you obtained is the result of your training. It wouldn't be as interesting if you only had 1 evolution. So, since Digimon are data, which can change drastically, let's consider each Digimon as a state, reachable by other Digimons depending of some conditions and, to make things more readable, let's limit each Digimon to 4-5-6 evolutions.
It's absurd to consider Digimon's evolution as darwinism. It is in a way because it's affected by the environment and education, but it's not done in a biological way. Pokemon is the opposite, evolutions are done in a biological way, but are not affected by the environment, with the exception of the regional forms, but they're more different species than Pokemon who evolved due to their environment. Their evolutions are still determined.
