5e D&D Requires less AC to "work correctly"

 I was wondering why I couldn't pick a +2 Ring of Protection on the DnDBeyond site. 

Short answer: AC is WAY WAY lower in 5e than in 3.5e, such that you can't give big AC bonuses or nothing will hit except on a crit. 

• Benefit is, little mobs can still do dmg, so they aren't 100% throw away (I like this. Has a Dark Souls feel... has a reality feel, too). 
• Suspicion - did they do this to distance themselves from Pathfinder, which was always a 3.5e rewrite? 
• Con is, you get less loot. Everything is "attuned, max 3 attuned items". I dunno, I like loot to keep coming through the levels. Starting to feel like once you hit level 10, there are no more decent loot items to get, outside of super duper items, or getting all the same oddball items (every char in every campaign going to have bag of holding, immovable rod, etc. ... boring). 
• Never get a +3 or +4 or +5 swords. I don't think they make em past +3, and nobody gets past +2. 
• Continual loot: 
• Pearls of Power, more is good. Never too many.
• Wands, scrolls, potions, never too many. 
• Gonna have to invent my own. 
• Energy Bow sure was cool. Gotta use extra abilities more, eh? 
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https://www.reddit.com/r/dndnext/comments/3ybaha/5e_ring_of_protection_2/

I think you've sort of hit the nail on the head there, though - the intent is that the hobgoblin should never be completely unable to hit the PC. The PC should still feel threatened by 20 hobgoblins. But saying "Making this ring of protection +2 or +3 wouldn't be so bad..." and then doing that with a few other things, suddenly the PC has 25 AC, and the hobgoblin can't hit without a crit.

The point of this "flattening" of bonuses to hit and bonuses to AC, keeping difficulty classes/armor classes below 30, is to prevent such things. Wizards doesn't want the level of a monster to be represented so much by the inability to hit it unless you're the same level as it - it's the inability to do enough damage to kill it at all. This lets you stretch things a little bit. You could fight a higher CR monster than your party, with the right strategy, whereas in the past editions you'd be down some magic items and some Base Attack Bonus and it's just too hard to hit consistently. Likewise in older editions large numbers of low CR monsters were no real threat - your AC was just way too high for them. In 5e though it's a little more dangerous.