The unpalatable truth:

1. Because most aircraft now weigh considerably more on the taxiway than they do on paper, and this suits the operator's bottom line.

2. Because nowadays there are very, very, few performance related accidents involving big aircraft or events where the fact that a big aircraft was slightly overweight was critical, and so...

3. Regulators and investigatory authorities never weigh big aircraft after accidents (though they often comment that small aircraft were overweight, which they often are).

That's one of the main reasons for the system not becoming widespread. If it can't be made exact enough to be the sole means of W&B you always have do the manual W&B anyway. In that situation, you save nothing but may end up cancelling a few flights due to finding out that while the load sheet said you were OK you were in fact above MTOW.

Airlines want to be able to load the aircraft right up to the limit. If the system cannot be made without safety margins larger than the manual load sheets, they will be able to load less. I e, until the system can be proven to have the same tolerances as manual calculations, it will not become widely used.

Another reason. If all the other airlines are using dated standard weights, airline XYZ will not use actual weights (on-board W&B) if it means they can fly less pax around than the competition.

Check out the Society of Allied Weight Engineeers' Recommended Practise for On Board Weight and Balance Systems.

True. Imagine what a stiff wind coming down over the roof of the adjacent terminal onto the aircraft will do to the indicated C.G.

In summary: Unless it can be built more accurate and with better repeatability than is currently technically feasible, it will not be commercially viable. Hence it is not likely to happen unless regulations change to require it.

They're essentially looking at acceleration monitoring during the take-off run. It would alert the crew to overweight or underpower conditions, but not to out of CG range. Not a bad idea IMHO. A simple speed check halfway down the TORA would suffice.

In the world of smaller aircraft operating out of short fields, I've heard it argued that every runway should have midpoint markings to facilitate such a check. Makes sense to me.

Cheers,
/Fred

Weight scale at bog.
 

You are right, there is a scale in BOG, ATC sends you randomly and it's a pain.