It is because lower frequencies have longer wavelengths and cause other objects to resonate, this forms a range of different frequencies which are sensed as more energetic and therefore louder to your ears.
When you are closer to these low frequency sources, they don't appear as loud because you only hear the pure sound wave, which can only be heard by the human ear above a certain frequency.
I experienced this at a custom car show, there was a car stereo playing techno music, it was hard to identify the source because all you could hear was the extremely loud bass, the whole area felt like it was vibrating, my body included. As we continued walking, it felt like we were moving away from the sound because it was getting quieter but you could hear the higher frequencies. We eventually found the car, the bass was no longer as loud and you could hear the full range of sounds from the speakers. I was shocked by the near/far contrast!