Electricity wouldn't scale. Solenoids on each brake system would take current. Adding new cars would increase the load on whatever is providing that current. To ensure that cars could be swapped in and out, the cables on all cars would have to be large enough to provide current for even the longest train.
Air brakes, once pressurized, don't take much to maintain. Relatively small pipes work on even very long trains. You can add cars all day without increasing the steady load on the compressor. A longer train will take longer to pressurize, but once there it would not need the compressor to run constantly. Air is also not as dangerous as running large electrical cables the length of a train.
I meant electricity just for the triggering, not for the action. A small electric signal could for example open a valve in the air tube. Thus all cars would break at the same time.
Some trains already to this, where they put in electronic valves in the air lines for in car. Basically, the electronic valve controls if the existing brake is vented or connected to the central air line. This still allows for the fail-safe behavior of being able to brake the train from the central air line by venting it.
This lets them start easier/quicker from a stop, as they don't have to re-air the entire train and less wear on the brakes of the trains in the front of the train as all the cars start braking at the same time. This is particularly useful for trains that have to start on a hill as taking minutes for your brakes to fully turn off can complicate things.
Passenger trains have used electropneumatic brakes for decades. A few freight trains use them. Trains with distributed power (locomotives spaced along the train) have sectionally controlled braking. Sometimes, there are "repeater air cars" with air compressors and a radio link to the locomotive. Those are used mostly on flat terrain, where more braking power is needed but not more engine power.
Because the pneumatic system is fail-secure, since a break in the line causes the pressure to drop and brakes to engage. I suppose you could have a system which required constant power to hold the brakes open, and loss of power would cause them to engage, which would have the same result?
