I think part of this is that while Viper, Cobra and Anaconda are known snake names also in other languages, Asp is not. Hence many people didn't even know that this was a word.
Secondly, maybe more importantly, did Frontier themselves spell Asp 'ASP' on their website. In an old version of the Shipyard subpage, there was the sentence that the 'ASP Explorer is the civilian version of the military model Asp Mk II' . I could imagine that it was spelt ASP also in other, 'official' places. So if even FD were not consistent in this, no wonder players are not... and, honestly, why wouldt it matter?