Probably because both הילל and שׁחר are known deities from Canaanite mythology. In other words, despite your scathing reply (matched in its dismissiveness only by the utter lack of anything remotely resembling an informed view), you've mistakenly equated speculation of a particular myth with speculation of references to Canaanite mythology. We know these are references to Canaanite mythology.Isaiah "may be" drawing upon imagery of a Canaanite myth? It "could be" then that Isaiah alludes to a lost Canaanite myth? If it was lost, how then does what was found have any credibility?
However, I'll try to make this simpler for you. Imagine a reference to Zeus in some ancient text. Imagine further that in this reference, Zeus is mentioned as "friend of Medea". So far as we know, there is nothing in Greek literature to suggest such a relationship, but if we discovered such a text we might hypothesize a particular myth/story about Zeus and Medea, yet be very sure that even if this hypothesis is wrong, it's BLATANTLY OBVIOUS that this text concerns Greek mythology.
But nice try. Disparagingly dismissive ignorance is always entertaining.