There are two clamp diameter standards for brake levers and shifters: 23.8mm for "road" (i.e. drop, but also bullhorn) handlebars and 22.2mm for "mountain" (i.e. flat or riser) handlebars.
Why? Couldn't we just pick one?
Or at least make all brake levers/shifters in the larger size and provide a (cheap and easy) shim to adapt to the smaller size. Oh, and the expander plugs for bar-end brake levers and shifters should be the smallest size possible with shims to fatten them out to larger bars.
Here's why: How frickin' cool would it be to be able to swap bars and brakes and shifters around willy-nilly, building your own bizarro Frankenstein's monster, then rebuilding it into something else? Flat bars with bar-end shifters sticking out the ends? Why not? Drop bars with hydraulic MTB levers up on the flats and GripShift down on the ends? Sure! The whole world of bike cockpits would become a giant box of Legos to be torn down and stuck back together however you want. I'd be SO into that.
I imagine that the "tuner" market isn't big enough to justify the chaos of switching to one standard. After all, how many cyclists know a 22.2mm clamp from a 23.8mm? Or even know that two different sizes exist? But still, a fella's gotta dream.
(Don't get me started on 25.4mm vs. 26.0mm vs. 26.4mm vs. 31.8mm stem clamp diameters, either...)