Why don't more Soldiers wear their full size medals?

I actually once asked the Air Force Uniform Office (via email) which uniform combination Airmen are authorized to wear full sized medals with. They said "none." The exception is the USAF Honor Guard (and maybe the Band), but for the average Airman it's shadowbox or nothing for the full sized medals.

Part of the brand!

Marines must wear full sized medals with regular Dress Blues. The SNCO tuxedo style Blues (E6+) wear miniature medals.

The Army doesn't pride it as much as the Marines do. In the Marines some of their balls or formal do require dress blues with medals if the command specifies it. I have attended quite a few balls when commanders made it mandatory for all NCOs SNCOs and officers to wear full medals and junior Marines were given the option to wear either Medals or ribbons and badges. Medals especially the anodized ones are expensive to buy, but its worth the investment. Every year with the exception of officers, military enlisted men and women do get a clothing allowance so it is affordable to purchase all the the full sized medals you earned. That's how I was able to buy my full sized medals. I wore full size medals on my ASUs when i got married and I also wore them to a father and daughter dance. They do tend to start a conversations because you will draw some attention if you have quite a few of them.

One reason is that there are few occasions to wear them. Another is the common misunderstanding that ribbons are the "correct" thing to wear when wearing your ASUs at a formal event whereas the Regs allow for miniature and full sized medals to be worn.

I did it once for a ball. It was cool and all for the novelty of doing it once, but I can honestly say from experience the two reasons I think more folks don't do it: time (effort) and money. It takes a lot of reading up on how/where/when they can be worn to get smart on it (which could be a reward unto itself depending on your mindset) and the suckers are just downright expensive. I believe when I did it (around 2002/2003) each full size ribbon cost me approximately $20. I don't recall specifically how many I sported (probably still have them all in my SWAG case somewhere) but guessing it cost me somewhere between $125 and $200 just to do it. This is on top of buying dress blues which weren't yet standard issue at the time.

I have minis, fulls, and ribbons and I wear them depending on the formality of the occasion. I've got a short list of guidelines that I've worked out for myself:
Weddings - full size
Ball - depends on the size of the event and formality of the venue - (brigade and up is full, battalion is minis or ribbons dependent on venue, company is always ribbons)
formal Dining events - usually ribbons unless otherwise stipulated
promotion - ribbons
graduation - ribbons or minis, depending on the attire of the other guests
funeral - full size
change of command - usually ribbons for uniformity purposes
parades - usually ribbons for uniformity purposes
orchestral concerts - minis
memorial/veterans day - whatever I feel like depending on where I'm going that day.

I hear y'all saying "it a cost issue", it is my understanding that you are issued the first medal and replacements as needed. Those of you still on active duty go see your Supply Sergeant, they should be able to help you out and those that have left the service write the Department of Heraldry.

No it's not required for Marines to wear medals in their blues during a ball but I know I did it because it makes the uniform more elegant and sharp and because of pride of looking our best

They get in the way. The bar ribbons look fine.