An echo is weaker than the original sound because the sound wave imparts energy to the surface from which it bounces. When a sound wave is directed at a distant surface, particles of air in the path transmit the energy to the next particle, until it reaches the surface. When the sound wave hits the surface, it transmits energy to it. This loss manifests itself as a weaker return signal.
An echo is strongest when the original sound wave strikes a hard, even surface. Soft surfaces and uneven surfaces absorb more energy, so there isn't much left in the return signal and the echo is weaker.
Additionally, the farther away the surface is, the weaker the echo. The intervening particles of air lose a little energy each time they transmit the sound wave to the next particle, so for great distances, the echo is weaker even when returning from a very hard, even surface.
If a surface is very close to the original sound wave, then there isn't an echo at all - but a reverberation. The leading end of the sound wave returns to the origin even before the trailing end has hit the surface, so a continuous sound is heard.