Or D) the Kaminoans saw the way the Empire was going and decided not to help them anymore.
At which Palpatine has the Imperial Starfleet (yes thats what the call it) blows Kamino to hell from orbit after a bunch Stormtroopers come by shoot up the place and steal the cloning technology, then shoot the place up somemore.
Unless the Kamino destroy there own cloning facilities first.
O.K., a lot depends on what you consider canon, but bear in mind that in the Thrawn Trilogy the capture of a functional cloning facility was a major boost to the remains of the Empire. This strongly suggests that cloning is by no means common at this time, certainly not on a 'churn out an army' scale.
Also, as I recall, when Luke encounters Stormtroopers on board the Katana, he detects something odd about them through the Force. Only after the battle does he discover that they are clones. This implies that he has never before fought clone troopers and been aware of it.
Now in ANH his Jedi skills were almost non existent, so we can't say anything about the nature of the Troopers he meets. By the time of ROTJ, though, he has become a Jedi Knight. He meets Stormtroopers on Endor and the second Death Star. Obviously we are not privy to his thought processes, and he is somewhat preoccupied with meeting his dad, so we can't be sure whether or not he notices something odd about the Troopers.
Nevertheless, in 'Dark Force Rising' it is presented as if this is the first time that Luke has been aware of the oddness of clones. Though we can not be certain, it does appear that the Katana incident is the first time since Luke achieved Jedi Knighthood that he has fought clones.