Why did the Empire stop using clones?

There isn't a definite answer, but there are a few likely reasons.

Phase I clone shown above

Likely Reasons: Genetic Disease, Kaminoan Rebellion, or Expensive

Genetic Disease

Making Clones from the same DNA over time would lead to health complications. Either they start getting really sick, unfit to be soldiers, or disfiguration. Over time this would start to create an issue that would be hard to deal with and they would be wasting their money. If they all started to get weakened immune systems, then there would be less soldiers because of rampant disease. If they have disfigures, then they aren't up to standard requirements and unable to fight or be all that agile. Any form of disease would force them to shutdown.

The Kaminoan Rebellion

Another ways they probably discontinued the Clones was the occurence of the Kaminoan Rebellion. The Kaminoan workers became unhappy with the way things were being ran, so they destroyed a lot of the machines, clones, DNA, and so forth. After a while they went back to work but did so unhappily. The Empire would have feared if they continue, the Kaminoans would program the Clones to attack them.

Expensive

And finally, it just became too expensive. Having to pay for the production plants, the production itself, housing, etc. Would definitely add up in cost. Although this may be an unlikely reason because well, the Empire could probably afford anything, it was worth stating.