Why do people tend to say the Kara no Kyoukai anime "does not exist"? It's clear from the anime that it exists. Some of the characters from the main series are there. As close as I can tell, it's just a side series.

So why do hardcore Type-Moon fans say with resolve that Kara no Kyoukai is not a Type-Moon anime or that it does not exist to them?
Kara no Kyoukai, like other Type-Moon series exist in as part of the same multi-verse. While not in  the same universe as Fate or Tsukihime, they exist as an alternate universe, like Fate/Extra and Fate/kaleid liner. The characters could in theory meet one another... if the circumstances are deemed right.

Canaan, however is not part of the this multiverse.