I didn't play 4E from very long, but wasn't Longsword on the short list of weapons that had +3 to hit, and thus better than pretty much everything else?
It's been a long time since I did the math, but I think higher proficiency bonus than comparable weapons was the advantage of swords in general in D&D4? That said, A straight up +1 was good through heroic tier, but as level increased it became less and less relevant while the other weapon abilities scaled better.
Again, it's been forever since I've thought about this, and my memory is foggy, so please don't call me a liar (about this, anyway).
My guess is because 5e is going back to a more simpler system. Judging by the feedback, that's what most people wanted, so I wouldn't consider it a step back any more than I'd consider Coke taking a step back when they went back to Coke Classic.
Eh. We wanted simple, yes. But simple doesn't have to mean illogical or poorly balanced. The loss of D&D4 weapon balance is /not/ a feature of D&D5. Balance in general was the albatross of D&D4, but when it comes to something like weapons you want a little balance. Playing a knife fighter ought to be as mechanically valid as a greataxe fighter. Otherwise you're just eliminating character options from the table for no apparent reason.
I mean, seriously, in the last five combat encounters in your campaign, how many suits of plate mail were there?
Just to play devil's advocate: I use a lot of hobgoblins, and by the RAW, their rank and file are in chain and their captains are in half-plate.
I've just always felt that the best interpretation of the "Armor Class" system is armor is impenetrable /in general/ (perhaps with the exception of critical hits), and increasing value represents greater coverage. If you roll an attack, and your result is higher than your opponent's AC, you are hitting them in a place that is unprotected by their armor.
If I were going to base combat success on armor penetration I'd use damage reduction, hardness, armor HP, and yes, damage type effectiveness rules. But that is so much management for so little gain!