I wouldn't call Bluetooth "relatively new". It's been around longer than the Legacy control system.
Legacy has far more operational capabilities for O gauge than Bluetooth. Bluetooth is not a replacement. Legacy was designed specifically for O gauge operations, with capability for complex operations. Also, the concept of integrating Bluetooth for simplified model train operation is relatively new. As far as a control system, the first IPhone with Bluetooth capability didn't appear until 2011, which was a very early version with limited capability. The latest version of Bluetooth just appeared in the past year or so. It is likely that early versions of Bluetooth did not have adequate capabilities, including range, to allow integration with model train operation.
First things first, Bluetooth is not a control system, it is a wireless protocol used to transmit data to and from devices. Even the earliest versions of Bluetooth could easily transmit the data packets required to a tmcc base and send it out to the rails. It's just 1 & 0's, that's all.
In fact, I've used a Bluetooth serial port receiver plugged into a TMCC base to wirelessly send TMCC commands to engines and TMCC accessories from a Bluetooth equipped laptop. This concept is not new, in this case it was an experiment to see if it would work. And it did, back in 2009! Again, Bluetooth is only the conduit used to send the needed data to and from devices.
Bluetooth is a universal communication standard used for Millions of devices. For example, We use Bluetooth equipped GPS receivers on our farm, not for guidance, but just for programming the receivers and entering unlock codes. Guidance is still handled by a good ol' wired serial port connection because Bluetooth is still too unreliable.
The only thing Bluetooth needs to do in the case of train control is send data packets back and forth, this could have been easily accomplish using the version 1.1. The problem is practical application for the technology, maybe a universal remote could have been made but as you pointed out smart phones didn't have Bluetooth until much later in life where the technology could really been taken advantage of.
Early versions of Bluetooth had good range. Bluetooth 2.1 would easily make 100 feet in an open area and about 30 feet indoors.