Velvia was the reason I found a Fuji developer studio in the first place - all cameras would take 35mm, but find a location which would develop it was a challenge of its own.
I'm not a big film photo geek (probably under 100 rolls over a decade), but once you found someone who could push/pull develop your colour film, that makes the choice of film sticky.
Came for the contrasts, stayed for the contrast fix-ups - once I found a place which did Velvia 50 & Ilford 50, that's all I bought.
In any case, both of those were niche films for pros and a few prosumers.
The way I saw it, Kodachrome was more for consumers who just wanted to press a button and have it done. And Fujichrome was for people who took the process much more serious. Because they could do more with the "inferior" product than relying on the one-size-fits-all commercial process.