Why were scabs unpopular with striking workers during the late 1800s?

They were unpopular during the late 1800's because a "scab" or "strike-bearer" is a person who worked during a strike. They can be individuals who aren't employed by the company prior to the strike, but are hired after or during the strike to maintain the operation of a company. In the late 1800's, strikes began because of wage cuts, machine inventions, and the depersonalization of workers.