dangerous dog act uk question

hello i need help with som facts about the dangerous dog act in the uk.
i realy having problems to find out what went down in the early 90's
in the uk

i need to know the history behind the law, the time before 1991 i have problems to find facts before 1991 and why they ban pitbull/fila/tosa/dogo argentino and all mixes of the breeds.

that i have found was the media start to report a lot about dogatacks in 1990 to 1991 and that lead to the law dangerous dog act.
but the attacks and bite accident dont increase att all att the same time
and the breed that was involved the most was rotweiler.

so could someone tell me why they ban those breeds and dont ban rottweilers
if someone have a link so i can read about it or explaine it to me
im confused about the law and want to know the background and why they chose those breeds

http://www.dogstrust.org.uk/information/factsheets/doglaw/presentlaw/dangerousdogsact/