Because there was finger oils on the eraser.
So: don't touch the parts of the eraser that you use on the paper.
(I had to learn this from comic creator extraordinaire Dave Sim (Cerebus). You'd think somebody would have known and mentioned it in the 500 books I have read about making art and the materials, but nope.)
Cerebus: Dave Sim's 3,000-page "comic book" about... God, the Universe, and Everything.
(No kidding. 300 issues. Took 24 years.)