I've been trying to figure something out, GR created Star Trek TOS and TAS right!
Why did GR decide TAS wasn't canon, who is Richard H. Arnold and what part did he play in the decision?
It was a rather political decision, and was also coveniently intended to avoid red tape and delays in approvals of tie-in manuscripts.
TAS was produced by
Filmation, in coordination with Gene Roddenberry's Norway Corporation and the children's division of NBC TV. DC Fontana was put in charge of daily production duties. David Gerrold contributed several TAS scripts.
Paramount had nothing to do with TAS, except for taking over distribution to the TV stations after its first TV run.
After a falling out with GR over Season One of TNG, DC Fontana and David Gerrold commenced a lawsuit over co-creator status on TNG. At the same time (1989), Filmation was being dissolved (partly bought out by Hallmark, IIRC), and the rights to all of Filmation's backlist of cartoon series and TV shows went into a state of flux. The first series of DC Comics' "Star Trek" had reintroduced Arex and M'Ress from TAS into the post-ST IV storyline - and for DC Comics to keep using those characters may have caused lots of time and money to be tied up in legal red tape, ascertaining if Paramount actually owned the rights to the Filmation-created TAS characters.
Also, Larry Niven's kzinti appeared in TAS (and the semi-licensed "Star Fleet Battles", not by Niven's choice) and the very real possibility of the kzinti returning to the ST universe (eg. the novel "The Captains' Honor" had to be slightly rewritten) may have prevented Niven from selling his "Known Space" and "Ringworld" stories to a roleplay game publisher.
(Speculation: Politically, the decanonization of TAS may also have been to set out parameters for the upcoming lawsuit: de-emphasizing the creative roles Fontana and Gerrold had played in TAS, to strengthen GR's case over TNG?)
Richard Arnold, a long time ST fan, friend of GR, and LA-based volunteer tour guide of the ST sets, had finally been offered a permanent paid job within the ST Office at Paramount after ST IV, with the title of ST Archivist. He was suddenly in charge of vetting all licensed ST tie-in proposals and completed manuscripts
on behalf of GR, taking over one of Susan Sackett's old positions. Because, with the sudden growth of ST product, due to the popularity of ST IV and TNG, the ST Office was concerned to newly-constrain the licensed tie-ins to just the parent TOS and TNG shows.
All existing tie-in licenses were declared null and void, and Pocket Books and DC Comics had to
renegotiate new license contracts. RA and GR devised a memo that specified that ST canon was to be made up of
live-action ST, as screened on TV or movie screens, and as produced by Desilu/Paramount/Viacom. No longer were the licensed tie-ins to incorporate plots, original characters or tech from other licensed tie-ins, such as novels, comics, RPGs... or, indeed, TAS.
Due to GR's passing in 1991, the ST Office's 1989 memo is no longer restraining the tie-ins; RA was fired from his job - and the first new TAS reference turned up in the novelization of TNG's "Unification"! Arex and M'Ress now appear in the "ST: New Frontier" novels. But Paramount now very clearly owns TAS and its characters, so the legal constrains aren't a problem. Except maybe for Niven's kzinti.