Why is digital "static" different from analog static?

Digital broadcasts are just a stream of digital information: a load of ones and zeros. 

If that stream of information isn’t good enough then the digital receiver simply has nothing to translate into sound and pictures.

But analogue broadcasts just display whatever they get. And if the only thing they get is the Cosmic Microwave Background radiation then you get static. The CMBR is an exceptionally weak signal and gets totally swamped by any normal analogue broadcasts.

for an analog signal there is no threshold level but for a digital one there is so the level less than the threshold cannot be useful

It isn't a matter of "static".    
                  
All digital TVs, and many analog ones, 
are Programmed to display a blank raster instead of one that consists of noise.   
Older analog TVs had no built-in video generator from which to obtain a blank raster, 
which is why they display only noise.    
By the way, approximately one percent of the noise you see when that happens 
is "cosmic background radiation".   If you don't know what that is, look it up.