I guess you ask why they are using water at ambient temperature (20°C; 68°F) instead of very cold water (0°C; 32°F). Some reasons I can think now:

They are using a lot of water, as most as possible, from pipes at whatever temperature it is. There are no enough mobile refrigerators, not enough electricity to make them work, and it's very hard to transport cold water or ice if you don't use the pipes.

Also, the center of the tank is hot and reacting, but the external part is a nasty block if plastic that acts like a shield and isolate it from the cold water outside.

This is a common problems in big chemical plants when you have exothermic reactions. It's not enough to cold it down, you need to ensure all parts are cold down.

For comparison, there is a nice video by NileRed https://www.youtube.com/watch?v=phNLecfyWS8 He is making Bakelite that is a type of plastic. It's a tiny amount, in a lab, on purpose and he may make a few attempts. Anyway it overheat and instead of a nice piece of plastic he got a nasty block of foam with burned plastic. No imagine a huge tank of a similar chemistry reaction.

They have been doing exactly that for the past 24 hours. However the contents of the tank are polymerizing, that reaction is exothermic, and the tank is quite large.

Read the article. They have been doing that, but that is just slowing things down and buying them time.

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