I think the question already jumps to conclusions which is not completly accurate. You could say this about many countries and ancient regions. Why was Greece, India, China, Egypt etc etc controlled by so many ancient empires. And to say that they were "easily taken" is not accurate. Remember that among ancient empires Persia (Iran) is among one of few that was not totally colonized or completly vanished.

When it comes to Persia - one needs to look at the geograpy. The challenge for Persia/Iran is that there is few natural borders. Persia could be attacked from east, west, north and south. The only protection of the heartland is the Zagros and Alborz mountains that give some protection against invaders. Therefore the strategy of Persia always was strategic depth - by invading or controlling neighbouring people and lands it gives the heartland protection against invaders.