When a user has a large variety of choices available to them like the previous talent tree allowed, sub-optimal combinations cannot be avoided. You cannot expect Blizzard to be able to tune the game so all combinations available to the player will result in a viable spec.
why not? this game's revenues are in excess of over $1 billion dollars a year. it's not like Fred, and Ethel are busy in the backroom coding, and we can't work them too hard.
Software development is not an assembly line. Adding a another eyeball to a doll you manufacture (because ... you're insane, I guess?) and adding another person to an assembly line to install that eye isn't how a collaborative effort like software - and especially game creation which includes a lot of art and "feeling" - is created. Some of the reasons are encapsulated in Brooks's Law
Anyway to address your "why not?" more directly, the difficulty in balance is really that we want classes and choices to feel meaningful and unique. When you play your warlock you want to feel like a warlock, and not like a slightly different mage. And so it's just a fact that uniqueness and equality are two opposing concepts (no, you're not a unique snowflake AND as good as anyone else at everything), and in that we have to find some middle ground we call balance. But you're asking why talents can't be truly balanced if there's choice, so let's draw on a piece of paper a grid 3x3 and in each we'll put a unique number 1-through-9. I'll pick 4 of those numbers, 6, 7, 8, 9, add those up, that's 30, and then you being the same class pick your own 4 numbers from the same grid and you choose 1, 2, 3, 4, that's 10. Those numbers aren't equal, and almost all combinations will not be equal. Obviously a very simple if not flawed analogy, but balanced and fun vs. unique and meaningful isn't a problem to be solved by adding more calculators to the pile because it isn't just about spreadsheet math.
But this is all waaaaaaay too off-topic.