Two reasons off the top of my head (I suggest you research to verify):
1. Serfs are one step up from slaves. When you have cheap labor, you don't need machines, so there was less of a demand for technology.
2. Serfdoms tend to be agricultural in nature and therefor less industrial.
On a side note that was why in the American Civil War the non-slave north had more industry and manufacturing (which eventually led to a Northern victory) than the agricultural, slave economy of the south.
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Two reasons off the top of my head (I suggest you research to verify):
1. Serfs are one step up from slaves. When you have cheap labor, you don't need machines, so there was less of a demand for technology.
2. Serfdoms tend to be agricultural in nature and therefor less industrial.
On a side note that was why in the American Civil War the non-slave north had more industry and manufacturing (which eventually led to a Northern victory) than the agricultural, slave economy of the south.