I've never understood why ALL bikes have only one side stand and on the same side.
That's an easy one to answer: Weight and space. Have you looked at just how small, light and optimised sports bikes are these days?
It's no just the weight of the stand but the strengthening of the mounting points to support the bike (with rider and pillion because someone will sit on the bike on the stand) while it rocks backwards and forwards on the stand. It also has to be mounted in a way the doesn't interfere with cornering while still having a suitable lever to ensure the bike can be put on the stand.
Gone are the days of roadside tube replacement so, for most bikers, there really is no justification for a centerstand. A set of cheap paddock stands allow home wheel replacement and chain maintenance so, for most, a centre stand serves no purpose.