Who says they can't?
Israel has a significantly large Iranian Jewish population. Considering that Iran and Israel didn't have relations at the time many made Aliyah, it should be understood that there had to be something for them to move en masse to Israel.
There are still organizations that seek to bring Jews to Israel from hostile areas. For example, a few years ago they brought a lot of Yemenite Jews home and it made some news. Iran is no different, and I know of some offers that were made to them to relocate. The community that remains is small yet well rooted, so they weren't willing to relocate; however, a lot of people (including Iranian Jewish ex-pats) do point out that with Iran's current governance they have to be vocal about rejecting it
If you want more information, I'd ask a general Aliyah organization like Nefesh B'Nefesh and see what they can give you. If you live in NYC or LA, there are HUGE Iranian Jewish communities who may be willing to give you more information. I can't be definite because a lot of things regarding getting Jews out of Iran can be rather secretive, as Iran is hostile not only to Israel but to the idea of their Jews leaving. Supposedly, entire families aren't allowed out of the nation even for a vacation.