Why doesn't Espeon evolve with a Sun Stone and Umbreon with a Moon Stone?

Question

It just makes sense doesn't it?

Answer 1

Most certainly, but there can be reason for it being the way it is. Bare in mind these are my own theories and aren't backed by any official sources.

First of, the Moon Stone existed in Gen 1. This might not seem like much but it is important. Throughout the generations and the addition of new evolutions new ways have been introduced to evolve old Pokemon into newer evolutions. What is important here is that no old Pokemon have evolved into a new Evolution using old methods. This is mostl likely to maintain a facade of new Pokemon being in the games when they weren't even invented yet. As in the case of Umbreon, the Moon a Stone existed in Gen 1, which would make it ineligible to evolve Eevee.

Secondly, the Eeveelotions have frequently been used to showcase new features in the games. Jolteon, Flareon and Vaporeon showed the evolution and branching evolutions concept, Sylveon displayed the new Fairy typing (Leafeon and Glaceon didn't really feature much new). So what did Espeon and Umbreon spotlight? For Umbreon, the new Dark type, but also the Day-Night Cycle. Back in the days this was an impressive feature, especially for a Game Boy Colour game. Espeon/Umbreon are Sun and Moonlight Pokemon respectively, so it seems likely that they were used to highlight this feature with their method of Evolution, instead of using traditional (and possibly lore breaking) methods of evolution.

Comments

About Leafeon and Glaceon:

Their method of evolution was probably to showcase Location Evolutions, which were introduced in DPPt, since Magneton and Nosepass both evolved into Magnezone and Probopass, respectively, when levelled up in a certain area.

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umbreon/espeon also showcased friendship evolutions

Answer 2

Espeon and Umbreon were probably introduced to take advantage of the time system newly introduced in Generation 2

Answer 3

My opinion is that it's much more fun and interesting than just giving it an Evolution Stone. It would be pretty boring if all Pokemon evolved with Stones, and I find that it's more entertaining to raise your Pokemon's happiness so that it can evolve depending on the time of day.

As Espeon is the Sun Pokemon, it makes sense that it should evolve only by day, and the same goes for Umbreon, the Moonlight Pokemon to evolve at night.

Sometimes, it is also much easier to evolve your Pokemon this way, as you often have to wait until you find certain evolution stones, or sometimes completely miss their location.