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

In the USN and likely therefore the USMV also, Full Dress Uniforms REQUIRE full sized medals. It is part of the Uniform Regulations, and yes, it is pricey, but you should have all the medals you have been awarded so the cost is all in the mounting.

Because they are a pain in the butt to mount if you do it yourself; If you get additional awards requiring additional devices, they tear up the suspension ribbon (ribbon replacement is only a buck and change, medals usually start around ten to fifteen bucks); Unlike the sea services, there is no way to display ribbon only awards...
Mounted mine before I got married, but did not wear them as my wife thought my ribbon rack actually looked neater on my blues. So now they sit in a wooden cigar box along with the rest of my bling that I had at retirement.

Wearing medals in the USMC is usually an option, but in the waters I swam in, no one wanted to be outdone by anyone else, especially by abl "boot " (junior Marines).

Because Full Size Stay Brite medals are expensive, that's why. My rack of 7 with oak leaf clusters and knots cost over $150 eight years ago.

Most functions I've attended where Dress Uniform was required the wear of the full size medals seemed to attact unwanted attention, more so when you don't want to steal the thunder from another individual being honored. I've only worn my minitures on a few occasions while in civilian dress and then they were secured afterwards. Perhaps, its also inregards to how after the Vietnam Era was viewed by much of civilian society that some of us choose not to be too obvious in Civilian events. Add that the joing Ladies and Men serving in our current conflicts need more recognition for their service than I do as there is now a greater disconnect from the wider society. Please by all means if you feel it proper to wear your medals do so. You earned the right. For myself, I'll stay out of the spot light and work behind the scenes where I am more comfortable.

Frankly, I don’t like how full sized medals look. To me, it screams “Central American dictator”. Others think they look great. More power to them. If told to, I’ll wear ‘em. If not, ribbons it is.

1. they are heavy. 2. you have to worry about getting them tangled. 3. You worry about losing appurtenances. 4- You look like a Mexican No War General, of a NK General . Normally for formal function I wear my mini- medals- much easier.

To address your question regarding Marines, they have an actual dress uniform, we don't. And what you're seeing is Dress Blue Alphas (blue jacket, medals, cat 1-3 badges and ribbons).
Since there is an offical uniform, that uniform can be called out as required with allowable substitutions. The other Dress Blue uniforms are:
Dress Blue Bravos uses blue jacket, ribbions and cat 1-3 & shooting badges.
Dress Blue Charlies L/S with tie/neck tab, ribbons and cat 1-3 bagdes.
Dress Blue Deltas is S/S with ribbons and cat 1-3 badges.

When I wear the four-in-hand necktie, I wear ribbons. When I wear the bow tie, I wear full-size medals.

Because thedamned things are heavy!!!