Why was Robert E. Lee not called Robert Edward Lee?

Answer 1

Many people in Lee's time, as well as the people who live in this time now, don't usually use their middle names in the way of presenting themselves to others or in conversation.

It seems much easier for most people to use the initial of their middle name.

I'll try to give you an example, Union General, Ulysses Simpson Grant, never used his full middle name either. He was and will always be known as Ulysses S. Grant.