The US has a history of major data breaches by low rank IT security personnel throughout multiple branches of the government. A security monoculture is not necessarily a great approach on the balance of things.
They do and did use many methods of communication and the pagers were just one part of it. Excatly to not be reliant on conventional computers and their insecurities, but seemingly easy and secure devices. They just did not expect the sophistication of the Mossad to hijack the production facilities.
