Two reasons:
1) Voice does switch off my podcast listening (Overcast). Very disturbing. Not happening with iSmoothrun.
2) Do not like voice, I prefer sounds as they can accomplish almost as much but in less time. Especially when I do short intervals like 30 secs on/off the watch is speaking most of the time ;-)
Edit: sound and haptics combination of course
If you use the haptics and not voice then watchOS automatically adds a beep as well if you have the watchOS alert volume above zero, so that may help.