Regarding the "previous adaptations", all previous films have portrayed Kaa as a villain, for the simple reason of how people tend to view snakes. This is a significant deviation from the book.
The book shows Mowgli's growth from childhood to adulthood over roughly 15 years, whereas any film can only show a short period. The animals all provide various role models for Mowgli, and in Mowgli's early years the other animals are more significant, teaching him basic survival skills and protecting him. As Mowgli gets older though, he outgrows his early teachers and their innate limitations on how they think. Kaa's intelligence, knowledge of the past and grasp of strategy make him Mowgli's most important mentor during his adolescence.
Kaa can be frightening in the book (most notably when hypnotising the Bandar-log), but is never evil. He is merely calculating and strategic. He cares deeply for Mowgli though, because Mowgli's intelligence makes him the closest thing to an equal that Kaa will find.
Since the films don't capture this, they all share a common problem of what to do with Kaa. The films have all featured Shere Khan as the major villain, and this necessarily pitches the film at the earlier years of Mowgli's life when Kaa is less significant as a mentor. All adaptations seem loathe to leave out such an iconic character, but all adaptations then seem to shoehorn Kaa into a rather disjointed role as a minor villain because he doesn't fit well with Mowgli's other mentors at the time.
Perhaps there was a "focus group" assessment that a giant snake should not be one of the good guys? If so, that's a shame. A useful life lesson is that not all scary people are bad.