The summer runs always struggle to find decent celebrities. In it's prime, Celebrity Big Brother aired AT THE MOST once a year (series 1, 2 and 3 were all 2 years apart) and featured no more than ten housemates.
Nowadays there are two series' a year and each one has up to sixteen housemates, so you are looking at 20+ people to sign up every year.
Also, the summer runs are going to have the lesser names because of the smaller viewing figures. If you are Katie Hopkins, Katie Price etc, you are going to go for the series which has 3m+ viewers, rather than the one that you hope is watched by 2m viewers - it's better exposure and more column inches.
To be honest, it's rarely the big names that make the show entertaining. They want to keep their head down and leave with the cheque and their dignity intact. It's the desperados who want the exposure and that create the drama in order to be remembered.