>>15550A few reasons
On top of Order 66, all of the Clones have other orders including Orders 2, 3, 65, ect which cover removing the sitting Chancellor and his cabinet from office at the approval of certain governmental bodies, as well as a number of other orders which could have forced the Clones into compliance with different wings of the military, possibly against the Empire in the event of a military coup. On top of that, the Clone Troopers need to be housed, fed, and trained until adulthood, which granted is only around 12 years because of their accelerated aging but its still a lot of money. Conscripts are nice because their parents pay the brunt of the cost for raising them, the military just needs to give them like 4 months of basic and that's way cheaper than raising a clone army. In addition, these clones all will retire from service eventually, and finding something to do with them would not exactly be easy seeing as they age so quickly that they wouldn't be a strong store of institutional knowledge like regular veterans and they never had a taste of civilian life so integrating them with the population would be hard.
Also the types of enemies the Empire was going to fight wasn't going to be the endless waves of droids or the few disciplined organic seperatist forces like the Deathwatch, it was going to be ragtag teams of guerilla fighters in the Rebellion. Clones would just be hilarious and expensive overkill for an Empire already blowing its budget on endless amounts of expensively big overkill, corners had to be cut somewhere (TIEs, ground troopers, ect).