Another reason (often ignored) is that the UK law is heavily bound by the Human Rights Act of 1998 (which is based around the European Convention on Human Rights).
Among many other things this prevents discrimination by public authorities on anybody resident in or entering this country. With a few exceptions, people normally resident here, or with permission to be here, or very often here illegally, are not usually subject to restrictions because they have committed a crime (once their sentence is completed). As a result, (and once again with a very few exceptions) people seeking to enter the UK cannot be placed under restrictions (such as being denied entry) just because they have a criminal record.
One of the main problems (and there are many) with the Human Rights Act and the ECHR legislation is that it extends beyond these shores to be effective worldwide. Essentially, it prevents the entire global population from being subjected to �oppressive European government regimes�.
The USA is less ideological and far more pragmatic when it comes to controlling its borders. It does not want foreign criminals visiting and makes arrangements to keep them out.