chat has alot of advantages over voice comm.
chat is asynchron, so one can always recheck last commands in chat log, e.g. when you was distracted at the time a command was sent, whereas , voice comm is relying on the reciever to get the command right when it was sent -- or he has to reaquire the lost last command.
chat is aswell way easier to ignore, if the reciever "chooses to" not be reliant on commanding; in this way , the problem of muting command chat recieves much less priority than for voice ...
way more stable and unaffected by technical issues (noise,overtalking ...) than voice, some people may even have performance issues due voice comm
not so much affected by language issues, accents, voice issues like stutter or similar problems ... many foreigners can very well send and understand written english, but have problems talking and understanding vocal english...
chat is way more solid and superior choice imho
aswell as chat commanding can be well supported by highlighting and other gimmicks , in ways , that cant be done for vc..