BRT is still reliant on using roads that cars are on and even if it were electric, requires recharging (Unless it's on a wire). So rail in this instance makes more sense as you get cars out of the way and you wouldn't need to charge it. If it's on a guided wire might as well make it a rail.

The infrastructure for railways is considerably more expensive to build than a BRT, even with the dedicated lanes required for it to be fast.

A middle ground are light rail and tram, but those tend to be slower than a bus based-BRT system as the cheap rails don't support speeds as fast as a metro or railway.

> BRT is still reliant on using roads that cars are on

no no no no NO. The whole point of it being Bus Rapid Transit (as opposed to just a series of buses) is that it ISN'T just using the same roads that cars are on.