Depends, one major reason seems to be that under the then unrest and new-world brain-wishing of the red Khmer most of the feable documentation in civil registers were destroyed and thus its for many quite an undertaking to prove that they exist and that they are also those they claim to be. Of course these are rather understandable basics for any document as ID. Add to that the general Asean attitude of ´if they dont know where/who we are they cannot tax me´ and it gets more complicated to sort out.
Once having seen a Burmese passport (in a flight of AirAsia), just issued, just out of curiosity, from a friendly fellow pax beside me, one can well understand why developed western nations have reason to suspect such thingies now that they even have electronic gates for border checks with those chip-embedded passports.