The official excuse has been that Brown has to respect the devolution settlement and can't comment on devolved matters. But - what's this? - it seems he hasn't had a problem with commenting on another devolved matter before now: the level of health spending set by the Scottish Government. This from PMQs in January last year:
"The Prime Minister: I gather that the Scottish National party does not want this issue to be raised in the House of Commons. I have some knowledge of the Royal Hospital for Sick Children. It has given service to the community over many decades, and it is valued in the community. Unfortunately, the rate of increase of expenditure on health care in Scotland is not now the same as the rate of increase in England. That is the unfortunate result of policy decisions made by the SNP."
I put in a call with No.10 earlier to see what kind of explanation they could give, but haven't heard back yet. In the meantime, I'll throw it open to CoffeeHousers: why can Gordon Brown attack the SNP government over health spending but then say he can't comment about their al-Megrahi decision? Answers in the comments section, please.