Because Cromwell was great at overthrowing a monarchy, but absolutely lousy at putting together a Republic to replace it. Cromwell's legacy was an expansion of freedoms and a weakening of the Anglican Church. It also neutered any future monarchy and prevented the establishment of an absolute monarchy a la France.
A Republic of Great Britain would be far preferable to the inbred twits in the House of Windsor/Saxe-Coburg/Hannover.