Answer:
Because he wanted the rest of the world to join in a war against communism.
Why did Reagan call the USSR an evil empire? (This is the answer/explanation)
Due to the world's growth of repressive dictatorship and poverty, Lenin's and Stalin's purges, cooperation with Nazi Germany prior to World War II, the gulags, the KGB, and the Iron Curtain.
The USSR slaughtered around 62 MILLION of its own citizens between 1917 and 1980. That didn't include the millions of individuals they massacred in Eastern Europe and other countries to impose their ideology on the populations they held in check. Evil? The USSR was indeed a "Evil Empire," in my opinion.
Thank you.