The argument would be that he wasn't wrongly convicted.
By modern standards it's obviously a stupid, unjust and immoral law that he was convicted under; but if he was correctly found guilty under the laws of the time, then it was a sound conviction.
Pardons are a general tool to forgive (in several senses) a crime. If you look at the history of pardons in the US for example, you'll find examples ranging from people who were wrongly convicted, to people who got one as a reward for cooperating in investigations, to people who simply had powerful friends.
Which is why I find that reasoning confusing - when you pardon someone, you just do it... and if you care to give a reason you can.
Perhaps it's different in the UK? Is this some limitation on the powers of the House of Lords?
'It is the standard policy of the Government to only grant pardons to those who are considered "morally" innocent of the offence, as opposed to those who may have been wrongly convicted by a misapplication of the law.'
It also mentions that in 1996 there was a pardon given as a "reward for information".
By modern standards it's obviously a stupid, unjust and immoral law that he was convicted under; but if he was correctly found guilty under the laws of the time, then it was a sound conviction.