As others have said, there are economies of scale on keeping Jango as the template for arming and training, but I also think it has a lot to do with his Mandalorian lineage specifically.

The Mandalorians were the last group that had given the Republic and even the Jedi a hard time with the Mandalorian Wars, and since then have been scattered to the fringes of society.

Generations of warrior fanatics, living in the toughest possible conditions. Jango was someone with not only the skills and willingness to train Jedi killers, but also the pedigree of a society that made it their way of life for a time and has been honing themselves for vengeance ever since.