They are more prevalent than you think, especially in places like ports and airports. But part of the problem is they are typically expensive for anything more sophisticated than a basic Geiger counter, like most commercial offerings start at $5k and can very quickly go into the hundreds of thousands or millions of dollars per unit. And they require a lot of training to use effectively.

At least in the US there is also a pretty substantial network of teams and equipment for providing radiological technical support in an emergency. Examples of this are DoE RAP and the USNG CST. Now part of the motivation for this work was that a lot of the techniques for actually doing a search are fairly primitive, but there is a major support network with most of the tools and skills ready to go 24/7. It tends to all be very low key and fly under the radar, mostly because even a hint of one of these teams being deployed tends to cause a panic.