I a sense it isn't 'dead', because people are still playing, restoring, preserving, and enjoying them, but no new onces have been built for several decades. As far as I can tell, they went into a precipitous decline at the beginning of the last century.

My personal theory is the rise of first ragtime, then jazz, then swing, then rock. Popular music became dominated by rhythm. And what are the two names for a reed organ? "Melodeon" and "harmonium."

Try playing Scott Joplin on a reed organ sometime. The results will not be pretty.

As I grow older and older,
And totter towards the tomb,
I find that I care less and less
Who goes to bed with whom.
--Dorothy L. Sayers