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Because they use(d) a reduced character set that included parentheses but not brackets.

See https://en.wikipedia.org/wiki/Baudot_code#ITA_2_and_US-TTY

There was a time before ASCII, you know...

But this only punts the question forward: why do they still use this reduced character set, when most (if not all) of their data is transmitted over the internet?

I have no idea whether they still use this reduced character set. But whether or not they do, this could still affect the protocol or stylebook, for the usual sorts of reasons when legacy systems are involved:

* News wires are large old distributed networks. There may still be old equipment attached to it that natively uses this encoding; or modern software may still be using the compatible encoding because it was never practical or cost-effective to have a "flag day" to switch over.

* Even if the technical limitation has been lifted, the convention may live on as part of the folklore of the field.

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