Why is Kant's basic principle of morality called the "categorical imperative?"

Kant's categorical imperative is called this because it is
something that, he says, applies to all human beings (or at least all rational beings)
at all times.

There are other kinds of imperatives, such as
hypothetical imperatives, that do not apply in all cases.  Those imperatives are
typically stated in terms of "If I want X, I must do Y."  By contrast, the categorical
imperative applies to us no matter what sort of ends or desires we have in
mind.

One definition of "categorical" is "being without
qualification or exception."  This is the sense of "categorical that is used here."  The
categorical imperative is an absolute imperative that applies to all rational beings at
all times.  This is why it is called "categorical."