Because it kills it's mate (husband) after they mate so it in a way widows itself and it's a black spider
Because she's hungry. Most spiders are cannibals and will eat each other if the opportunity arises. However, the only Black Widow species where the female has been observed to eat its mate is...
because the female eats the male after breeding
Black widow spiders are called such because the females eat the males after mating with them.
On poptropica, the black widow is (sorry for spoiling the surprise) the investigator in front of the art museum.