Whats the difference between an Premiere open show and an open dog show and why arent there many about.

BOB at a Premier Open show qualifies for Crufts in some breeds. 

There aren't many about because even those who qualify to apply for one do not see the point. Most dogs wining BOB will be qualified for crufts anyway.

It is a very long and complicated application to apply to hold premier shows, not all clubs want to go through such a process.

We did it for 2 or 3 years but it is hardly worth the trouble for the show society or the exhibitor.

I agree that those winning BOB at a Premier Open will almost definitely have qualified already.
We got BOB last year at Bakewell, with an entry of over 60 it was still a really nice result to have, and it is such a well run show that we look forward to it each Summer. 

As far as I understand specified BOBs  qualify for crufts as well as breeds with 3 or more classes ( again BOB)