The energy loss is low is because of the low frequency/longer wavelength phonon - which does not coincide with the size of the molecules/atoms. The higher frequencies/shorter wavelength phonon is likely to coincide with the size of the molecules/atoms, and thus losing its energy through resonating the energy through all the molecules/atoms. This is the cause of energy attenuation.
Another factor is energy dissipation: By having the size of molecules/atoms coinciding close to that of the phonon, the phonon streams is more likely to be reflected/refracted and thus dissipated.
Both of these factors can help to explain why longer wavelength can travel far, low signal loss.
Another possible explanation is the phonon density: higher frequencies phonon matches with that of smaller atoms/molecular structures, which occurred at a higher density/number, and thus is able to spread the energy faster. Lower frequencies/longer wavelength need larger molecular structures (or multiple atom forming a macro-structures), which occur at a much lower densities, and thus is less able to spread the energy faster.
Another possible dissipative phenomena is electron-phonon coupling, which is more likely to happen for higher frequencies phonons.
These are my layman's perspective of what's happening, but from a specialist point of view (beyond me), u can refer to:
http://www.iop.vast.ac.vn/theor/conferences/nctp/proc/35/153.pdf (on resonance)
http://ocw.mit.edu/courses/chemistr...y-ii-spring-2008/lecture-notes/23_562ln08.pdf (which correlate the wavelength of the phonon with the size of the molecule/atom)
and
http://www-ee.eng.buffalo.edu/faculty/mitin/Papers/115.pdf (on electron-phonon coupling)
http://www.iue.tuwien.ac.at/phd/smirnov/node53.html
http://www.uni-tuebingen.de/meso/ssscript/phononen.pdf
http://ndl.ee.ucr.edu/Paris-Lecture-05.pdf
