I have a scarring on my face from road rash. why am i getting pimples only on the scar tissue?

It's normal: For probably unclear reasons, that injured skin is more susceptible to developing pimples. Be sure that the products you use to cleanse and moisturize are non-comedogenic. If they don't say they are, assume they aren't and that they will clog your pores and create problems.

Inclusion cyst milia: Inclusion cysts are quite common after a resurfacing injury. While a moisturizing agent to promote epitheliazation is usually recommended, excessive use may promote the formation of these cysts or pimples. Occasionally other reasons like persistent foreign bodies or herpes outbreak may also be responsible. Discuss with a physician.