Because your voice is bouncing off the walls. When there is carpet and furniture in the room your sound is assorbed.in to the object in the room when it does this it (your voice) doenst have far to travel before reaching the walls it bounces off the furniture,etc in the room. Take the objects out and your voice travels further hitting the walls bare floor and it makes it sound like and echo because there is nothing there to bounce the sound or asorb the sound off of.
Old question, but it has an interesting answer; to rephrase the question, "How far away must a sound reflecting surface be so that you can hear an echo":
The velocity of sound in dry air is approximately 343 m/s at a temperature of 25 °C. Therefore, the reflecting object must be more than 17.2m from the sound source for echo to be perceived by a person located at the source.
(Quora)