I wrote a fairly long answer to this here. It doesn't cover everything, so let me know if you have any questions!

shlin28's post linked to in his post in this thread is excellent. Interestingly, this territory is being covered at the moment in The History of Byzantium Podcast. Largely the same conclusions:

• Exhaustion after the Roman (Byzantine)/Sassanid War
• A "middle east" (our current term, not theirs) that for about a generation had not been part of the Roman Empire, and so with tenuous loyalty.
• Religious strife on top of that (Monophysitism vs Orthodoxy; Jews maltreated under Roman rule, but treated well under the Persians) creating a populace not pining for "the good old days"
• The Arabs were very familiar with both the territory and it's weaknesses, since various Arab tribes fought on both sides of the conflict
• And let's not forget that the fervor of their brand-new religion gave the Arabs superb morale, as opposed to their enemies.

I find this a very interesting topic.