Cable routing and why is there no optimum solution This is a general question, regarding the routing of the cables of both brakes and derailleurs on a frame.  

I find it highly practical to mount all cables along the bottom of the top tube. This offers the following advantages:  


The wide down tube protects the cabling from mud and gravy, originating at the front tire  
The top tube can support weight e.g. sitting* there or supporting luggage.


What are the reasons of the other cabling schemes? I am most puzzled the combined scheme "2 cables on bottom of the bottom tube + 1 cable on top of the top tube".

* one of my pleasant habits is, when there are no free seats in the park and the grass is wet, sitting on the front tube, with the bike tilted, one hand on the brake and the other holding a beer
What are the reasons of the other cabling schemes?


Most Cyclocross bikes will route all the rear cables along the top of the top tube. In a cyclocross race, you will get off and shoulder your bike, the cables would get in the way on the underside of the top tube. Many CX bikes have a flat bottom top tube as well for easier shouldering. Routing the cables along the top keep them cleaner too, cyclocross races get muddy and if they put the cables on the bottom of the downtube, they would get very wet, muddy and allow grime into the housing.