The simple answer to your question is cost. Much cheaper to do steppers than a servo drive. To take that further steppers are more than capable of driving an X3 so hard to justify the extra expense for little or no benefit.
Resolution depends on the components of the conversion and a whole lot of other factors. Typically I am seeing in other posts a resolution of 1 to 4 thou if ballscrews are used. But there are different ballscrews and some have zero backlash and some don't.
Adjustment of the gibs will have a influence on backlash and particularly so with the Z axis.
Mounting, coupling, bearings are also variables in the final resolution.
So it is difficult to answer your question directly but I hope that gives you an indication.
If you want a theoretical resolution, assuming zero machine backlash, stepper driver board on 1/8th micrstep, 5mm pitch ballscrews and 1.8 degree steppers then resolution will be 0.003mm.