I would be careful about describing it as 'easy'. It was won with blood and toil, and the Romans and the Persians certainly did not give up quickly! I outlined the main factors in this answer here, but it is by no means comprehensive, especially as many things in this field are still contested. I haven't read enough about Persia to write a comprehensive answer, but I noted some historians' arguments here. You might also be interested in this essay, which outlined some of the conceptual challenges about the conquest of Persia.

If you have any questions, I'll be happy to answer them, or you can post in the current ongoing AMA here, as we have a few experts on early Islam there who are happy to help!