[5E] ELI5: Why would Wall of Thorns be Conjuration, but Wall of Stone, Fire, and Ice are Evocation spells?

I'm having a hard time understanding the principle difference between conjuration and evocation in regards to elemental spells: several fire spells are evocation, and several are conjuration. Flaming Sphere springs to mind.

Can anyone ELI5? I'm actually a fairly new player.