Why is the mouse cursor slightly tilted and not straight?

The mouse, and therefore the mouse cursor, was invented by Douglas Englebart, and was initially an arrow pointing up.
When the XEROX PARC machine was built, the cursor changed into a tilted arrow. It was found that, given the low resolution of the screens in those days, drawing a straight line (left edge of arrow) and a line at a 45 degree angle (right edge of arrow) was easier to do and more recognizable than the straight cursor.