Even though I don't speak a language with linguolabial consonants, it seems to me that these sounds are easy to produce and also auditorily quite distinct, e.g. the difference between bilabial or alveolar plosives and linguolabial plosives sounds greater (to me) than the difference between voiced and voiceless plosives. So why are they so obscure?
Is it mainly that it's hard for these sounds to evolve from other sounds? I guess it's hard to think of a mechanism whereby another sound would evolve into a linguolabial consonant, except maybe dental fricatives which are already rare.