Because there is probably a well-defined regulatory framework for accessing data collected by the TSA, whereas there are few or no requirements when the same data is purchased from a broker.
It is not even certain that the data actually comes from the TSA. It could come from airlines, payment companies, etc.
There is no guarantee of quality when purchasing data from a broker.
The regulatory angle at least explains part of my wondering. I'm not really surprised that they have access to this information, I'm just surprised that they buy it, rather than just demanding it be handed over.
Buying has no accountability, no judges. Probably not even a proper paper trail.
They're spending public money, so the cost doesn't matter to them either. With this administration they can get unlimited funding.