Because with complex numbers you are implicitly defining a point on the complex plane. So how on the Cartesian plane you can have 2x + 3y, and that defines the point (2,3), you can have 2 + 3i on the complex plane defining the same point.

If you want to rotate the Cartesian coordinate -90 degrees,

    x' = x*cos(π) - y*sin(π) = -y
    y' = y*cos(π) + x*sin(π) = x
    2x + 3y => -3x + 2y

So check it out:

    (2 + 3i)*i = 2i - 3

It's the same as rotation!