Verified answer

Because a project like that would cost too much. The Empire only has so much money available for things like that. And with that pesky Rebel Alliance blowing up TIE fighters in record numbers, they have to spread their funds over several different defense projects. The only way they could afford to build 50 death stars would be if they raised the taxes, and that would only serve to aid the rebels in recruiting more people to their cause. Besides, blowing up planets wouldn't help the Emperor at all, he simply uses the fact that he CAN blow up planets to keep people obedient. If he went all George Dubya all of a sudden and started blowing up unnecessary planets, he would be ending the lives of millions of taxpayers. And then the idea of 50 death stars wouldn't even be possible. that, and he has to spend a bunch of money on the writers who come up with all that cool stuff for him to say.

It takes a long time and a lot of people to build a death star.

They started production on the first one at the end of Revenge of the Sith and it wasn't operational until A New Hope - several decades later.