First of all, Studio Pierrot released a lot of 
their genga/douga.

Take a look at all the Sayuki genga out there.  
I'm hoping they'll be as liberal for some shows 
they did.

Some studios were never really liberal about
releasing their cels to start with.

I guess there are studios that figured that while 
cels were cool, gengas probably wouldn't be. For 
the longest time, the overwhelming vast majority 
of studios felt like cels were just industrial 
waste.  TBS made a crapload of cash by having 
faires to sell off cels of shows they produced.

Also, a number of animators keep their gengas for
future reference. I guess it comes in handy...
Sometimes animators will publish doujinshi 
collections of all their gengas. 

I wish the studios that were really cool about 
releasing cels would be as cool with releasing 
gengas!