Perhaps because the bluetooth tracking is a lot more accurate. But since not even Apple can access the data (or so they claim), this shouldn’t be useful for surveillance.
UWB and Cell tower tracking are much more accurate for the majority of real-world applications
Now consider the vast number of “smart nodes” out there (eg street lights) that have multi-protocol connectivity built into them. Whoever controls these nodes can simply scan and log every BLE advertisement that is nearby.
I’m not saying that this is the only way that a person can tracked or that this kind of tracking is definitely happening right now, but it’s a very real possibility.