Why are all Scott Joplin rags written in "flat" keys?

Question

Flipping through my book (the COMPLETE rags of Scott Joplin), I notice that virtually every rag is written in a "flat" key--usually B-flat or E-flat major. Music theory nerd though I am, I cannot find a suitable explanation for why this is. Is it merely the author's preference, or is there some technical reason as to why Scott Joplin's rags--and presumably all other ragtime--is always written in "flat" keys?

Answer 1

Just a guess but I find improvising on the black keys to be much easier.  I've noticed that Lionel Richie composes a lot of music in flat keys as well.