What made mass production relatively easy in many industries?

To describe why it was easy means to describe how it was done:

Example:

So you want to make shirts:

You hire 12 ladies to sew shirts

Quickly you find if each worker does a certain thing only, for example sewing on the buttons or the left sleeve, onto each shirt that person can become very fast at doing that one thing.

Once you learn how to break every job down into small pieces it makes it easy to do a mass-production system.

For our example: So 12 ladies go from 24 shirts a day too 64 shirts a day, this lowers the cost of each shirt, so you can sell many more and make more money to boot.

You hire more Ladies and now you are a mass producing industrialist at the begining of the industrial revolution.