I might be all wet*, but I wonder about the relevance of bivy bags in an era of ~1# (and lighter) solo tents. One would choose the bivy for...cost reasons?
*and bitten by insects
There are reasons why someone may prefer one or the other. A few things I like about my bivy:
- It is cheaper
- I don't need to carry a pole
- It is a little lighter
- It works well for sleeping on hard surfaces like under a pavilion roof (no need for stakes)
- I can sleep on top of it and climb in as needed (I like to cowboy camp)
- I can pitch a tarp or not depending on the expected weather