You’d have to add significant new capabilities to pull off surveillance, which would increase the risk of detection.
Probably they did. The problem with pager like communication is that it is very easy to employ codewords. Something like "if we send orange, bring your weapon and meet us at the market, if we send pear pack a week worth of stuff and meet us at the pumping station, if we send melon disappear to the countryside for two days". Monitoring that can give you a sense that "something is happening" but it won't tell you exactly what. So all in all it is not terribly valuable.
But if I would be working for Hez I would assume that all communication over the pagers have been completely compromised.
The eavesdropping played a significant role in the decision to detonate the devices.