Because it's labeled as an illegal branch of Islam that deviates its teachings.
Personally, I would like a secular Iran.But it shouldn't be mistaken with 'Anti-Religion' Iran. You can't separate Iranian society from religion, but I think it's better to have a law based on non-religious views,hence rights of people from all kinds of beliefs is preserved.
I prefer a Secular Iran that respects rights of every single Iranian with any kind of belief: from hardliner Muslims, Christians, Jews Zoroastrians to Atheists and Non-believers. No one should be prosecuted for his beliefs as long as it's not hurting the society or people.
Current problem of Iran is not the system, Rules or Government. I think sanctions are the real problem. I do agree that more freedom can be given for other religious groups.
In my view Iran is doing exceptionally well and showed great achievements. Unlike our politicians and political system Iran has much better system and legislators.
Many of Iranian legislators are really patriotic, many even fought bravely against saddam without caring for their life. This shows that they are ready to sacrifice anything for their country. I am sure very less politicians in the world qualify this.
You have one of the most honest president in the world (Hugo chavez is another ex). May be some of his decisions you may not agree. But he is really simple, logical, honest and wants to bring fwd his country. He is not living in palace, not at all corrupt. not wasting countries resource to his personal gains.
As i remember you have more educated people than us, this means that your current system focuses more on education as well.
Reason for above are you are having a religious system, which is giving a correct result. Then why need to abandon it?
Why Iran has the strict rules to restrict freedom? Because people attracts to evil easily. What iran considers as modesty and decency west consider opposite to it as symbol of modern.
It is also fact and US openly claimed many times that it spends lot of money covert operations in Iran. So Iran fears most of the western items and try to forcefully block it.
