Honestly, because it's not an issue and no-one yet really thought about it, most likely. Attunement is related to the reduced brackets as well - and it's not as big an issue as detractors make it out to be, not by a long shot. It limits simultaneous access to certain things, but doesn't prevent the greater majority of what most folks like and want in their magic item system from earlier games, where it doesn't cause problems for the reduced bracket design.
The most common complaint you hear about it is:
"in 5e they've made magic items a lot more limited, and you're meant to have fewer of them" - Not true, not even remotely. You can absolutely trek around in your circlet of blasting, night-eye goggles, gloves of thievery, dwarven plate (+2, force move protection), +3 shield, boots of elvenkind, cape of mountebank, ring of animal influence on one hand, ring of wishes (last resort...) on the other, bag of holding at your hip, holding your box of marvellous pigments, while you fly towards the dragon on your carpet of flying, full in the knowledge that if it knocks you off you've still got your broom of flying strapped to your back as a back-up, but you're still deciding whether you want to get its attention with your wand of missiles first, or whether you're just going to fly straight at it and have a goo with your dragon-slayer long-sword in one hand and your +3 dagger in the other... or maybe if you want to just hang back and pepper it with enchanted arrows fired from your +3 longbow... and you know that even if it all goes wrong you've still got your rod of security in your belt, for a quick escape. And all this time you haven't actually used up ANY of your attunement slots yet - so you could still actually attune three more potent attunement items if you wanted, provided you can find somewhere on your person to hang them.
The availability, or scarcity, of magic items is, as it has always been, in the hands of your DM and the world space you're playing in.
