Explanation: Well, one reason is, since the army was so big to protect the empire, they couldn't keep up with paying all the soldiers taxes. It was too large to control and there were lots of barbaric wars going on thanks to the vastness of the empire. The economy weakened because they didn't have the money to pay the taxes as well. In the end Diocletian (emperor) split the empire to control it better.
Hope this helps! :)