I don't know if age or availability is a primary determiner, but looking at the LEGO Shop at Home site and the number of sets you are allowed to purchase, and I get these as the most restricted sets with only 1 or 2 allowed for purchase at a time. TB is still at medium throttle like the rest of the sets with a max of 5. I'm kinda new at this and haven't had the opportunity that some of you have had watching exclusives go on sale, retire, and appreciate over the years, and I'm probably trying to see too much in the data ...
Item
Theme
Title
lego max
Retail
Year
10188
UCS
Death Star
2
400
2008
10234
Advanced
Sydney Opera House
2
320
2013
10237
LOTR
LOTR Tower of Orthanc
2
200
2013
10240
Star Wars
Red Five X-Wing Starfighter
2
200
2014
42030
Technic
VOLVO L350F Front Loader
1
250
2014
71006
The Simpsons
The Simpsons House
1
200
2014
75060
Star Wars
UCS Slave I
2
200
2014
76023
DC Universe
The Tumbler UCS
1
200
2014