I presume that you mean the Green Card Lottery ? If so the reason given on the USA Immigration website is
'The Green Card Lottery, officially called Diversity Immigrant Visa Program is a US government lottery for people who come from countries with low rates of immigration to the United States. Your country (United Kingdom) has received over 50,000 Green Cards in the past 5 years, therefore you are not eligible to participate in the Green Card Lottery.'
The other ways of obtaining a green card are:
Marriage to US Citizen
Through Employment
Through Investment
As a Special Immigrant
Check out thier website for further details:
http://www.immigrationagency.org/