For example before the Refinery update we had:
They were removed temporarily for reworks and fixes I think, however I don't know why they couldn't leave them up in the beta while they do so.
Same with some of the clothing items among other things that have disappeared.
Like Berggman said. For reworking, etc. Also I think they just don't want to release all the content in the Beta already, to make the release more worth it and actually make it a 'Release'.
This Beta is just a technical test, not the full game.
I believe the devs are also rotating the available maps to gather more information about a particular set so that they know how to improve them.
Okay that's great to hear. I thought it could have been a bug or something.