One of the major reasons was that there were quite a few people who were happy with the status quo and didn't want it to change. They might have had existing business relationships or connections to Great Britain and didn't want those things to be upset or changed by a revolution and changes that might come after the conflict.
Another reason was simply loyalty to the Crown and to Parliament, feeling that they were the legitimate authority and ought to be obeyed. This was a view shared by many who were labeled "Tories" or Loyalists during the Revolution.
Others simply felt that there was a good chance that the Colonies would lose the rebellion bringing down further taxes or other negative consequences and felt that simply going along in order to appease the Crown and Parliament was the best way forward.