Speaking from fairly limited experience, as to "why", I think it was to cut down on unwanted resonance from the relatively thin steel "funnel" of the horn.
I have three large ( 30 inch) long horns:
> panelled Tea-Tray Company "morning-glory" horn ( tin-plated steel, very thin )
> Superior all-brass horn
> Hawthorne and Sheble "Silk-Lined" "Black and Brass" horn ( Brass bell, steel cone with black silk jacket ).
The Tea-Tray Co. was my first big horn, and I noticed that when playing music with loud passages and in certain musical key-centers, the horn itself would begin to resonate with the music- confirmed by lifting the reproducer off the record during such a passage, and hearing the horn continue to "ring" for a second or two afterwards.
For me ( a musician with a fairly sensitive ear ), this can get to be pretty distracting.
One day I had a talking machine friend with good ears over, and I demonstrated this, and we did a simple comparison experiment between the three horns, checking for sympathetic resonances.
We found that the TTCo. horn had the most [unwanted] resonance, the all-brass Standard horn very little resonance, and the H&S "Silk-Lined" horn no detectable resonance.
I think another factor was manufacturing cost - it was probably least expensive to manufacture the "black and brass" style horn with a thin steel funnel, and throw a fabric jacket on it to kill any unwanted resonance.
That's my little theory.
