BRT has notable disadvantages; buses can move from side to side unlike rail, so the tunnels and bridges have to be bigger or use proprietary guided technology like this one. In general road surfaces wear away much faster than rails. And rail vehicles perform much better once you factor in labor costs. BRT is useful in some cases, like where there is no dominant trunk line and buses can leave the roadway to serve different destinations; but eventually the common trunk gains enough critical mass and you convert to light rail anyways, as Seattle and Ottawa have discovered, and LA is considering for the Orange Line BRT.