Bicycle riders generally can output about 1/4 horsepower. (I know, everyone here can do more that that, but we're far better than average... stick with me and you'll see the point) Chain drives can be well over 98% efficient. Shaft drives are doing well to be 85% efficient.

So with only 1/4 horsepower, can you really afford to lose 12-15% of your output to an ineffecient drivetrain? People spend an enormous amount of time and money trying to overcome even 1% of other losses like rolling resistance and wind resistance, 12-15% is quite a bit if you put it in perspective.

And then there's weight. Shaft drives are heavy.

Az