I'm not a Boeing pilot but if you look at this pic (
http://cdn-www.airliners.net/aviatio.../5/2225594.jpg) you'll see that the
'box' the thrust levers are on is right in the centre. The thrust levers are offset because of the trim and speed-brake levers to the left of it. It makes sense to separate the speed-brake lever and the flap lever so I guess someone decided it was best to put the trim lever on the left side and that was that! Someone who flies Boeings might be able to provide more insight.
By the way, on jet engines the 'throttle' is called a thrust lever(s), on turboprops they're called power levers, and on piston engines they're called throttles. The definition of a throttle is "a mechanism by which the flow of a fluid is managed by constriction or obstruction." In a piston, the power is controlled by restricting the amount of air the engine can get. If you open the throttle, it'll mix with more fuel and the engine will increase in RPM. In a turbojet (very loud but used for very high and fast airplanes), turbofan (modern airliners), or turboprop (essentially a turbojet with a prop on the front) [all three are gas turbine engines], you control the engine by adding more fuel into the burner can which burns hotter/faster (harder, better, stronger..... haha jk) and that faster rushing hot air now goes through the turbine section of the gas turbine which turns it faster which is connected to the compressor at the front which sucks in more air and compresses it further which is good for the burner can because then it can add even more fuel and get the engine spinning even faster. In the end, you accelerate the particles (air and fuel) through the engine faster when you push the thrust lever forward and that creates more thrust.
