Answer: There were fewer people to fill jobs.
Step-by-step explanation:
The plague led to the death of millions of Europeans and as a result, deprived them of a very valuable commodity, labor. Europe's economies were shattered as the number of people willing and able to work to produce goods drastically decreased in the number.
The ones that could work had to pick up the slack and demanded a higher salary for doing work that only they could do and so began the practice of labor negotiations in Europe.