Why?

Probably because most of Western society is descended from Ancient Greece and the Roman Empire, which Persia was enemies with.

Oh I thought it was the Assyrians who were

ALWAYS? Or in Hollywood since 1979?

If anything, Iran had been basically left out in terms of discussing its place in great civilizations, mainly because it went its own trajectory and continues to exist - a spur, a side branch - as opposed to a tidy linear line that lead to the pinacle in England then the USA, at least as far as Western History has been written and disseminated. China too had been dealt this card to some degree, at least until recently.

But for years "Persia" connoted Eastern mystique, the exotic, far off land of spices and carpets.

Since the Hostage Crisis if 1979, however, tarring its image has been in vogue. They made the convenient go-to terrorist to avoid upsetting portraying our allies who've made that a cottage industry as such, and The 300 ushered in its past as fodder for the bad guy on the block in the ancient world.

fodder to be used as*

They were? Like Babylonians and the like?

It isn't completely undeserved.

Except that they were remarkably tolerant of other races, religions and even sexualities. Women could do things (unlike Athens), and military service wasn't required (unlike Sparta), so it was a truly equal society. They had a flourishing economy due to their placement on the silk road, so their coffers grew full with revenue. If a group surrendered without a fight, the only requirement was that they pay taxes to the Persians. They had a powerful army, and their system was so efficient that when Alexander the Great conquered Persia, he used the exact same one. Their only flaws were a lack of democracy and their attitude to groups that did revolt.

Which is why the first Messiah of the Jews was Cyrus the Great?

Last anyone heard, the rise of the Greeks was directly linkeded to their aggression which did not bode well for Iran, deliberately destroying the country), and Rome wasn't excatly renowned as the Red Cross either. Heck, the latter even turned Christianity into a vehicle for conquest and aggression of a sort not seen till recent centuries with the rise of colonialism.