Why did it take 26 years to make that happen?
I can speak to my observations.
Like never once have I ever, ever, ever seen players walk away from a great game session, glowing and heaping praise saying things like, "Oh man, those mechanics were SO unified. Like WOW! Oh, and when we were battling the BBEG. When he used his special attack and the GM said to roll the exact same thing we've been rolling the whole session? I nearly creamed myself right there!" It just doesn't happen.
I have, however, seen times, for example, where the GM calls for the use of the grappling rules when a hound attempts to lock its jaws on a character. And that contrast from the usual combat mechanics really drives home the feel that you're not just fighting another character with a wolf's description, but rather you are engaged in combat with a beast that fights differently from a man. And I absolutely have seen players praise that sort of thing after an intense encounter.
There is a sense in which D&D always had unified mechanics. Maybe not one rule to rule them all. But two.
Rule #1. Assign a probability and dice against it.
Rule #2. When there are co-dominant variables, use a matrix tookup to determine what you need to roll.
That's it. That's the whole game in two rules. I'm not quite sure when "mechanic" became a synonym for dice. Just because a dwarf rolls a d4 to detect a new construction hoping to get 1-3 and d6 hoping to get 1-4 to detect sliding or shifting walls does not mean those are two separate mechanics. You're just simulating probabilities.
Rule #1 is quicker and simpler than the modern D&D mechanic. You don't have to figure out how to express the probability to conform to the d20.
Rule #2 is quicker for those who are slow at math, slower for those who are slow at lookups.
So from a simplicity perspective, it's not clear to me that the "modern" way is an improvement.
And then I think about things like the 1E Strength table and the probability of forcing open doors or lifting gates. It's rule #2--a matrix look up. Here's your strength. Here's what you're doing. Okay, here's what you need to roll. If you look at it, there's not really a strong pattern to it. The only way I can make sense of it is, it's as if they were constantly applying rule #1, asking the question, "Okay. What's a reasonable chance for a character of this Strength to complete this task," and then running with it.
The key is, at every step you're asking "What is reasonable?" Not "What makes a pretty pattern I can write a sexy formula to?"
So from a cohesiveness perspective, again, it's not clear to me that the "modern" way is an improvement.
I don't deny that there is a certain perspective--a very common perspective--that streamlining the rules makes them easier to use. Learning and applying the rules can be a burden in a sense, and streamlining is about minimizing that burden. But speaking for myself, when I read through the 1E DMG through some obscure rulings, I find they inspire me. They're so oddly specific, they kick-start my imagination. To streamline them would be to generalize them--to make them generic. And that strips them of a lot of their power to inspire.
Okay, so my opinion differs from these other people's opinions. Big whoop, right? It's all subjective. Then I ask myself, would I rather game with a GM who cracks open a game book and tends to see work and complexity and burden and things that mess with fun, or one who cracks open that same book and tends to see imagination, inspiration, and all the things that make the game fun?