This is because there are many towers but only one gets the denotation "Tower of London". The words that set it apart come after the common part. In "Buckingham Palace" does not receive this because the more specific part of its name comes before the more common Palace. Without "the" before "Tower of London", we would need "a". It would then sound as if a person were speaking of one of many towers that may be in London.

This question occurs periodically. Here is a related discussion. https://www.italki.com/question/487599