Why is evidence placed in a paper bag or an envelope rather than a plastic one?

It isn't always put into paper bags/envelopes.

They are only put into paper bags/envelopes if the sample needs to dry or avoid degredation for rotting.

For instance a pair of blood stained jeans, they would be placed in a paper bag so that there is a bit of air flow and the jeans will then not rot.

However may other sample types would normally be placed into plastic bags, such as fingermarks lifted from a scene. these do not need the air flow so are placed and sealed into an airtight plastic bag.