Maybe because t1 and t2 did not happen in that timeline anymore. Didn't they use some t1 left over parts to make sky net in t2? If they don't have those parts it takes should take longer to make. I did not watch the tv show so no idea what they did, is the tv show considered cannon still? Is it worth a watching now?
*edit* When i first wrote this post, I misunderstood what you were saying. You are absolutely correct, the lack of T800 parts from the original terminator is why Judgement Day was pushed back so far.
That's my point. In that timeline, destroying all of Cyberdyne's research and the leftover T800 parts still only pushed Judgement day back from 1997 to 2004. The creation of Skynet is basically a causal loop. In T2-3 and TSCC, Skynet is always developed out of the research done by Cyberdyne related to the leftover T800 parts.
Incidentally, I just answered my own question. The reason Judgement Day was pushed back so far in the Genisys timeline is because Sarah and Pops completely destroyed all the parts of the T-800, leaving nothing for Cyberdyne to research. Skynet was forced to rely on sending itself back in the form of John, which is, amusingly, another causal loop. I guess it's not a predestination paradox because of the way time works in Terminator. By sending itself back through John, Skynet is essentially creating a new nexus at the point that it sends John to, causing the timeline he sent John back from to immediately become divergent.
Honestly, the more I try to pick it apart, the more I like the movie. If it had been rated R with better chemistry between Courtney and Clarke, it would have been just as good as either of the first two.
As for canon, nothing but Genisys is 'canon' now, because Genisys is now the main timeline, and all the other movies and shows are divergent. TSCC was always a divergent timeline that picked up after T2 and assumed T3 never happened. Judgement day happens in 2011 in the tv show.