Why must the line of origin be drawn in pencil instead of pen on the chromatography strip?

Because if you use pen, your chromatography will separate also the colors of the pen and not just your sample.

Explanation:

The pencil since is carbon as simple element, is solid and is not dissolved in solvents like commonly are used in paper chromatography, it will not interrupt the separation of the compounds in your mixture, otherwise, your results will be confuse because you are not sure if the spots that you observe is due to the ink pen or due to the mixture that you want to separate.