When you think about it, Espeon should be the most common eeveelution in the Pokemon world.
Assuming that most people with an Eevee are not trainers and just have an Eevee as a pet, but even among trainers:
Vaporeon, Flareon, and Jolteon all require a specific stone, they don't occur naturally. Someone with an Eevee would have to go out of their way to intentionally evolve them into one of those. And unless they are planning to (and have the cash for such a stone) there is a good chance an Eevee will evolve by other means before that happens.
Glaceon and Leafeon even moreso, depending on the region you either again need a special stone, or go even more out of your way to go to a very specific special area and train your Eevee there.
That leaves Umbreon, Espeon, and Sylveon. All of them evolve with high friendship (or affection depending on the region), these three are the most likely to happen if someone simply has an Eevee as a pet, or even a trainer who is not intentionally trying to evolve their Eevee to any of those previous five types. Sylveon has the additional requirement of also needing to know a fairy-type move, so again, unless intentional there is a good chance they will not know one at the time of evolution. That just leaves Umbreon and Espeon, and since humans are generally more active during the day the chances of triggering the friendship/affection evolution during a day instead of night are much higher.
Simply put, unless a trainer is intentionally going out of their way to evolve their Eevee to something specific and does it before it maxes out friendship/affection, or someone simply has an Eevee as a pet and is not training it or trying to evolve it at all, the chances of it eventually becoming an Espeon are much higher than any of the other options, with Umbreon and then Sylveon being the next two most likely. None of the others will happen on their own without the trainer/owner intentionally going for that one and putting in work/money.
