AkuyiIn short, it's a design choice. The avatar system is modeled after RPG and dress up games. It was created to be clean, sleek, easily comprehensive, highly functional, and fit the theme of Windlyn. The equip slots keep items and layers where they should be and offers easy to use, themed layouts. I wouldn't really call the system "unable to handle it", but rather the system was specifically designed to work a certain way, and not any others. If I were to change it to allow for multiple equips of the same item type, I'd have to throw away what I've been building the past few years.
There's glitches here and there, sure, I'm still learning a lot as I go; and there are definitely a lot of pros and cons to both systems, but we've forked off from where we could do infinite equipping a long time ago.
I agree some items would look good together-- I have a hard time picking the categories to put some of the items in (and you're always welcome to suggest a better category for an item if I put it somewhere). However, for a vast majority of items, they don't look good together. It seems unfair to users and somewhat nonsensical to change the system to work in a way that works for 10% of the items, and doesn't for 90%.
I hope that answers your questions. The idea of equipping multiple items has been proposed a few times now, and it's on the back burner if a way to incorporate it smoothly does come up.
Thank you again for the suggestions. I do appreciate it.