Complicated language is often about the writer. It's self-centered. YOU want people to know that YOU are smart. YOU want to express yourself in a way that makes the most sense to YOU, even if it makes less sense to your audience. YOU want to show off what you learned over the years, even if no one else cares.

Simple language is about prioritizing the listener over the speaker. Try to make the message as easy for them as possible. The point of communication should be ensuring that the audience understands.

It's not condescending, it's about efficiency. People might be able to understand a 5,000 word essay from a very articulate person who likes to show off their personal lexicon, but a 500 word essay with easier-to-digest words is preferable because you get the same message across with less time and effort for the reader.