Unfortunately, the question of why a purple dragon would be flightless and have a venomous sting may never be answered. The first illustration of a purple worm appears on page 5 of Monsters & Treasure i suspect that some artist read the Chainmail entry, perhaps out of context, and was inspired to render this image, which may have in turn inspired Gygax to invent the new monster he had inadvertantly named. It is likely that between the publication of Chainmail and that of D&D, someone made use of a 'Purple dragon' as a literal worm due to a simple misreading. It appears to me that the use of the term 'worm' here is an archaic term for dragon - appearing more commonly in today's fantasy literature and games as 'wyrm.' Tolkien used the term in his description of Smaug, as I recall, and probably in other parts of his Legendarium I seem to recall Poul Anderson did as well in various stories. as typified in Tolkien's THE HOBBIT.' After the rules are laid out, we have a paragraph in historical note: 'Other kind of dragons can be introduced into games, if a little imagination is used.' White, Black, Blue, and Green dragons are briefly described in recognizable form, then: 'Finally, the Purple, or Mottled, Dragon is a rare, flightless worm with a venemous sting in its tail.' The entry for Dragons in the Fantasy Supplement (p. Since nobody else has answered your question about where the purple worm came from (at least not here), it seems to be the result of a misreading of Chainmail.

D&d purple worm