Processing capacity. There's a reason why the system is generally limited to six or so people on the dais at any one time, and why the system is reset with every successive transport. The solutions where they use the transporter buffer explicitly only work because no one else has used that particular transmission system between the problem and the solution. And even then, the patterns stored in the buffer degrade over time.
The transmat systems of humans and many other species used in Doctor Who that use base stations as opposed to the "open ended" beams of higher civilisations are actually better/worse: in the expanded universe of the New Adventures novels, it's made clear that they work by beaming a person's quantum blueprints through space/time where matter is used to assemble a duplicate on the otherside, at which point after error checking occurs the original is disintegrated and recycled to make the copy of someone coming through in the other direction. Leading to people not using the system because of the suicide/murder of themselves involved.