Top mounted shift levers do not permit routing the cables under the bottom bracket shell, which was much cheaper, once the injected plastic router was developed. The plastic router is lot less expensive than brazed-on fittings or a router clamp and it takes less labour to install. It's also much easier and quicker to install cables routed under the shell. The one advantage of top mount shift levers is the single boss versus two, but the other savings of side mount shifters outweigh this. So, side mount shift levers have advantages both in terms of both materials, labour and after-sales ease of maintenance.