I'm assuming you're asking why BP is measured in volume and modern is measured in weight? The simple reason is that BP is field loaded, and back in the day, volume was much easier to measure with than weight, not to mention that weighing something in the field would be impractical!
Modern powders vary in density significantly, and are much more powerful, so a smaller mistake with modern smokeless powder could lead to overpressure and exploding actions. Black powder doesn't generate nearly the pressure (force that propels the projectile) that modern powder does. Being able to pre-load modern rounds makes it easier to accurately weigh the powder charge. It also allows for using one measuring tool (the scale) to measure multiple different powders with different densities. That being said, modern powder throwers work by volume, but you must set the volume by weighting the charge it measures. Once the thrower is set to the correct volume for that particular powder charge, you can measure your powder charges repeatedly using volume.
I've "double loaded" a muzzle loader before, 180 grains of FF in a TC Hawken, and other than rattling my teeth, there was no real repercussions! I'm not sure what the modern inline MLs are capable of, but the Hawken can handle a lot of powder!