Because, during the evolution of the thread, a few posters made comments that were critical to a certain engineer/personality who is the owner/founder of a digital modeling company. Specifically, the criticism is related to this individual's childish public behavior and his chronic trash-talk of his competitor(s), in order to puff up his company and his achievements -- always at the expense of, and belittlement of a competitor's creation, innovation and solutions. The latest example is his public claim of patent infringement with implicit threat of lawsuit.
The other forum (TGP) has an owner, as well as moderators, that are very protective of said individual (and his company). Hence the complete deletion, purge and memory-holing of the thread.