Answer
An echo is weaker than the original sound because the sound spreads outward spherically, spreading the original sound power over increasingly larger areas.
At the wall from which the sound reflects, the intensity is already weaker than at the source, and the weakening continues as the sound travels back toward the original source.
Add to this the fact that the wall absorbs some sound (it is not a perfect reflector), and it's no surprise that an echo is weaker than the original sound.
