Basic economics... A few buyers seeking a scarce commodity, i.e., first quality tandems.

A quick check of Ebay will yield about 10 decent 2nd hand tandems, two of which are NOS from Santana's home shop -- Bud's Cyclery in Claremont, CA. The four or so others -- Kuwahara, Trek T100, Schwinn Paramount -- and the Claude Butler are almost collector items. The rest are, well, they are what they are.

As for those who tried tandems and did not "click", I suspect a good number of those tandems were long in the tooth or lower-end models that continue to show up at from garage sales, on LBS bulletin boards, and local classifieds and I suspect some of the better models are held onto by the owners in the shop that they will be used at some point in the future, e.g.,the 7 year-old tandems that have hardly been used and that show up now and again. As for upgrading teams, while many "first" tandems pass on to new teams, a lot of them are kept as second hand tandems.

Finally as for sizing, with the exception of Bilenky who offers 11 stock sizes and Cannondale with 5, most tandems are only sold in 3 (S,M,L) or 4 (S,M,L,XL) different sizes vs. the 1cm or 2cm increments found on road bikes and the stoker compartment sizes are, good or bad, fairly standard with only minor differences across frame sizes. Thus, sizing is not really more complex; in fact, it's somewhat easier... well, filled with more compromises to be accurate.