This is how Gamefreak made the Pokemon. Espeon, Sunflora, and Volcarona all have a big connection to the sun. Espeon has to level up during the day to evolve. Volcarona is said to be born from the sun. Sunflora is a sunflower. There are other Pokemon like this too.
They are not linked in any way at all excluding the fact they all have a connection to the sun.
Not exactly Gamefreak logic, but rather how they made the Pokemon.