Because we did not come here as invaders.
And the fight with Islam continued here. Even with our reduced numbers.
I see it as a foot trying to stamp out a fire.
Embers still remain.
A fire rages elsewhere.
A vengeful fire.
But the embers of the good fire hold on. And it's those which will bring peace back to Iranian lands (not just modern Iran) as the bloodlines come back to the true path of Asha.
Against the backdrop of our history and three world empires, 1000 years is just one empire lost.
Persian is a continuation of Avestan. It's not a foreign tongue. Just an alien script. Which is why Zparsis will never accept it. And over the centuries never used it for our holy texts.
And eventually moved away from it altogether.
Only the Iroons still speak Dari. That too a codified version to escape the Qajjars.