Two things because the person is already dead pardoning them isn't going to do a lot of good for them is it? Furthermore the governments opinion that convicting them was wrong is already apparent due to the fact that they changed the law. Next, England is one the oldest governments on Earth being roughly 1000 years old (give or take a century), individually pardoning every dead person who committed a crime that is no longer a crime would be a major burden on the parliament taking time away from addressing the needs of living people. As a result it makes a lot of sense to have a blanket (and longstanding) rule; no pardons for dead people.