Why has this bacteria become a superbug?
a strain of the bacteria had acquired a rare combination of traits. Bacteria typically are either highly resistant to drugs or highly virulent. This strain was both.
Why is it so difficult to treat?
it is resistant to the antibiotics used as the second and third lines of defense against most superbugs.
Even Colistin, the antibiotic largely abandoned in the 1960s for its toxic side effects that has become the antibiotic of last resort for superbugs is not effective in treating some cases.