This is a long shot but here goes nothing. I think you meant to ask "Why does Tangela evolve by knowing Ancient Power?" I'll cover some factors to help you understand why, or at least why I think, why it evolves knowing that move. First, back in Ancient times there was a lot less civilization and more forestation. Wildlife and plants grew more frequently, with little to no harm, other than an animal eating them of course. The name "Tangela" is a play of the word "Tangle" which often results in someone falling. In most movies rather it be about a jungle or an Ancient Civilization, vines can be see around, covering ruins of a lost village or impeccably placed as a means of a getaway. Regardless of the movie, ranging from Jurassic Park, to Indiana Jones, when there's any mention of something ancient, you best believe there will be some vines. "Tangrowth" is a play on the words "tangle" and "growth" With the presence of vines more abundant during ancient times, this might explain why it evolves into Tangrowth and perhaps explains the reasoning behind the name, other than it growing in size of course.