Because...it's just a good idea, you know.
Besides...as a civilian I'm not into defensive weapons of sub-par accuracies when it comes to defending my life or the life of my family. I'll take the accuracy of any gun I own over the accuracy of any Tazer-like weapon any day, regardless of the range.
And by "sub-par accuracies", I'm talking about the inherent accuracy problems with a weapon which must get two probes, with wires attached, actually ON the target in order to be effective as compared to the accuracy inherent with a handgun which essentially hits exactly what it's pointed at when you pull the trigger.
This link has some revealing information on hit/miss and probe spread for Tazers. Start with page 48 and read through page 66:
https://www.ncjrs.gov/pdffiles1/nij/grants/222769.pdf
At 5 feet, the probe spread alone averaged nearly 10 inches for the Tazer. The Stinger and New Stinger cartridges were about half that.
At 10 feet, Tazer probes were about 19 inches. Stinger and New Stinger about half that.
At 15 feet, Tazer probes were about 26 inches, with Stinger and New Stinger about half that.
See where I'm going with this?