Probably becuase they helped cover it up. Katyn was invesitgated by Red Cross and neutral country representatives when the Germans found the main burial site, and the investigation was pretty clear in its results of Soviet guilt. However, the allies were unwilling to support the German position as that would threaten the alliance against Nazi Germany - they probably felt winning the war trumped the truth in this case.
After the war, I guess the US and the UK figured they would look stupid or duplicious to change their story, and so neither country made a big deal of it. I think I can understand not making a big issue of it during the war, but it was an injustice to keep quiet about it afterwards.