My guess is.....  most of the cel dealers collect 
or had collected cels themselves and they got 
their stuff from japan. 
Cel collectror are all the same, if one show is 
very popular in japan, it would be so in the 
states also. If many people in japan are after 
special scene or character, american cel 
collector would do also. 
Unless you have a dream cel from very very 
unpopular show, like me, there are many 
people who want the exact cel.  I am still 
looing for my dream cel, I posted here several 
times but no reply, asked mandarake japan 
but they don't have anything in stock. I never 
see my dream cel in any dealer's site.