It’s easier to encapsulate everything you want in 1 spec when establishing the class fantasy. It’s the reason why they said there’s only 2 Demon Hunter specs. Which was something along the lines of, they felt that coming up with 3rd spec for DHs would have diminished the feel of the other 2 and whatever they came up with might be incompatible with the total class fantasy they were trying to achieve overall.
Much like with mages - Blizzard has said they liked in the past when Mages were more adapt to using all different kinds of elemental magic. Fire spec’d mages used frost spells and arcane spells and vice versa. Now, they’re too geared towards making X specced Mages using strictly X elemental spells.