It's the same reason I can't get one of those awesome viking helmets in Norway.
I pretty much only have the "special" ears like Disneyland's 50th and WDW's 40th. My dad got Pluto Ears for the Halloween Party this year. I admit that they have gotten better this year!
Subsequent visits? No, thanks.
My second set of ears are my College Program ears that look exactly like the ones pictured, but have a tassel on them. My fellow CPs and I wore them from the time we got them through the end of our CP whenever we came to the park as a guest. I also wore them in the airport on the way home (and found some other CPs heading back to Jersey that way), as well as when heading to baggage claim to greet my Mom and my then-girlfriend who surprised me by showing up.
My third set are the "Graduation Cap" ears that I got when I went back later that year for my College Graduation trip. I would have worm them for a while at the parks, but we raced to get them on our last day and just barely got them embroidered on time to catch the group's bus back to the airport.
I still have all 3 sets of my Mouse Ears hanging in my room, and all three have my name embroidered on them. I wouldn't mind adding to that collection either with some special ears.
The first pair me and my family were awarded whilst queuing up for Soarin' (along with everyone else in our loading bay) - a special edition pair for the Year of a Million Dreams. They are sky blue with fluffy clouds on and the ears are silver. (many people have sinced asked where we got and have they can get them too!)
The second pair are very good quality, navy blue hat with silver ears with a load of bright blue LED lights in them. We rocked up at our villa and found them under one of the beds. When nobody claimed then, I took custody and love wearing them as soon as it gets dark.
I too have mine hanging in the house, in the front room. We have also started a new tradition of all wearing ears when we board the Disney cruise...! Really gets everyone in the spirit (even the male members who initially refused to join in!)
I think my pink leopard print Mickeys ears, and sequin silver Minnie's are my favourites....I have light up ones too for when the sun goes down....