Supposedly because other OSes than Windows could not run the streams (originally meant for Window Media formats).
Well, that was simply horsesheet because VLC (available for almost any platform) and even Amarok could play them fine without any problems.

I remember playing Fire Emblem Gaiden on GFCEU on Ubuntu 8.04 LTS while listening to YLE's Parasta Ennen radio program on the said stream/s.

Flash was much bigger cancer than what windows media ever was. Now the whole thing has been deprecated and good riddance!

Though i do miss the simple multi-media works and fun stuff created by people on various image boards / communities. R.I.P. /f/ /flash/ etc.