The engineering elegance of a motorcycle, being able to not only balance itself at speed, but to go around curves in perfect balance, with every part of the machine leaning and twisting its frame into the direction of the curve
and then with the slightest pressure on the grip the bike leans over the other way on an S curve
or with the twist of your wrist the machine speeds up, straightens out and stands up, pulling into the straight-away.
If you can ride a motorcycle, why would you tack on a third wheel and totally destroy every aspect of that experience?!