Jaguarxk120,
As already explained, it's for the same reason people climb mountains.
FrankenMauser,
I'm wondering if maybe the .251 was a typo and they meant to have a "7" in that last decimal place. Do you see otherwise-identical .257's listed?
44AMP,
I'm starting to think that finding someone with rifling equipment that is tooled for a rifle-length barrel in this diameter may be a hopeless cause. I looked through the CIP data, hoping some odd 6.35 mm might show up, but no luck. What I don't know is whether factories making .25 Auto pocket pistols or NAA, which makes them for the 25NAA cartridge (aka, 6.35 Fiocci), buys or make long blanks to whittle down to the little short pistol barrel lengths. You could call and ask. If so, perhaps they will tell you where to get a blank or sell you one.
One alternative might be to take a .243" barrel and spend a lot of time lapping it out to 0.250. A lot of work, but it could be done. The trick is to lap the grooves and rifling at equal rates, but there are ways to make that happen within reason. At least the .243 starts close enough to the same rifling land height (0.003" vs 0.0035") and should work to turn the low velocity bullets OK. The short bullets won't need as much twist as it will give (10" vs. 16"), but over-stabilized is better than the opposite and probably won't hurt much at the velocities likely to be obtained.