Stritations in a discharge tube

I am reading about A cathode ray discharge tube and I still do not fully understand why that as you keep on lowering the pressure in the chamber it creates striations, Faraday's dark space and Crooke's dark space and why it does not just stay as a constant stream of light? Also why does the gas have to be at a low pressure in the first place for the constant column of light to appear?

To verify my questions are:

1. Why when you lower the pressure to a certain extent you can see striations, Faraday's dark space and Crooke's dark space and why doesn't it just stay as a constant column of light?
2. Why does the pressure in the chamber have to be very low for you to be able to see a constant column of light?