Someone could probably find an announcement or something from wotc explaining it, but it's a split from the old ways. In the past the d&d rulebooks were written in a style that could be described as a form of
technical writing. With 5e they made it a point to avoid the added difficulty of technical writing & just write in a grammatically correct style that is often unclear confusing or downright useless for conveying technical things like the rules for a relatively complex exception based ruleset.
TL;DR it's a bad excuse
Eh, I'd say they had some justification. Large number of keywords and magical phrasing adds a lot of complexity to a game. Yes, experts and masters of the game favor it, but that's because
they've already done the memorization work. Experts love games that have fixed values that are used repeatedly everywhere, even for new content. However, the more time goes on, the more difficult it is for new players to understand anything at all. As more keywords are slowly added, it's difficult to tell what's important and what isn't. You can read rules and have no idea not only what they do, but what the author was even trying to express. All the meaning is buried neck deep in jargon. That's fine if all you want to do is sell stuff to the exact same group of people, but it's not fine if you want to grow your market.
One of the criticisms they identified with 3e and 4e was that it could often be inscrutable, and you could very easily miss important details because of keywords. They could be defined only in other books, or sound similar to other keywords, or be completely useless in 99% of cases. By abandoning the narrative elements of the rules for plain mechanics, the books read like they're 300 pages of nearly identical charts and tables. The common term for that is "eye bleeding".
The other trouble with strictly mechanical rules is that it turns into a rabbit hole. You end up thinking, "They need a general rule for fire so that we know, as a rule, what burns, how fast, etc." That's just insane. It's fire. It should behave like fire. You shouldn't need to designers to tell you how fire works when (a) you already know as much about fire as the designers do, and (b) there's a referee at the table already. No designer wants to sit down and physically detail an actual physics engine in pencil and paper games, especially if they're going to be criticized for how realistic or onerous their system is to use.
That said, I agree that it's not an excuse for poorly written rules. The stealth rules are basically, "Stealth is opposed by perception," followed by "these situations grant advantage or disadvantage," followed by several long-winded paragraphs where they try not to be too blunt that the actual rule is "you can hide when the DM says you can," and then says to the DM "stealth works in whatever way makes the most sense for that situation." Those rules are fine -- they do run afoul of things like Naturally Stealthy which explicitly don't follow common sense -- but they're fine for most RPGs. However, it's difficult to tell that's what they actually mean without stopping and reading the paragraphs several times, cross referencing the "hiding" sidebar, cross referencing the vision and light rules, reading the specific rule for darkvision, etc.
The problem isn't with narrative rules or natural language, it's with poor organization,
empty language, and not being plain and straightforward with the reader. The trouble is they
didn't want to write rules for everything but they didn't tell people that so it just confuses them when they don't find it in the book. They should just say they're trying to encourage you do just do what makes sense and that their alternatives are either (a) rules so complex you won't use them, or (b) rules so abstract they will be so unrealistic that they break suspension of disbelief. So just make something up that seems reasonable (probably with these skills), roll a die, and move on. The designers can't see your table from Renton.