Why does pulling out main speakers from wall improve sound?

Sound radiates out and around each speaker until it bounces off the nearest hard vertical surface. If you have treated walls, it can be mitigated in the upper octaves. Too close to the front wall, and side walls can cause too much lower octaves.

The room is small, about 13.5x13.9x8.

In my treated room, the speakers are less than a foot from the front wall, and 3 feet from one side, 4 feet from the other side wall.

Because of the treatment, I get a true 3D soundstage if the recording is done well.

About half of my collection is recorded well, or very well.

PS, my speakers are deep, about 18". So the drivers are all about 30" from the front wall.

Simple answer: Because they are designed to work that way. Mostly due to the increased use of subwoofers the speakers no longer need the bass reinforcement of a near wall or near corner set-up. 

The port in the back is for bass. The port should be away, no doubt, so that the bass does not become boomy. I also use a speaker that does not have a pass port in the back. But you have to realize that the speakers do have tweeters and mid-range sounds coming from the drivers. They first hit the side walls and the wall behind you and then reach the front wall - from where they are again reflected back to you. My belief is that the more time it takes for these reflections to bounce off the front wall (wall behind the speakers) and then come to you, gives you more perception of depth. By "treating" the front wall, you are killing off some of them. But you do not want to kill all of them and have a dead sound. Hence some folks have reflectors/diffusers instead of absorbers on the front wall.

This is in layman's terms. Someone more technical might be able to provide a better explanation.

Speaker placement is very critical, and can make a huge difference in your room. Get a roll of tape and start marking the floor. Get a base line for your room size and start. Do you have any room treatment in your room at all. That is a huge rabbit hole to go down.