The mass demonstrations and protests in Iran after the allegedly forged presidential elections of June 2009 were on a much bigger scale than what happened in Syria after Macrh 2011.
Despite the participation of up to 3 Million people on some days in Tehran alone and despite the disproportional use of lethal violence by the security forces the „green revolution“ ebbed away after a few weeks.

There are several reasons why the protests did not turn into an armed rebellion but the main factors differentiating Irans „green revolution“ from the Syrian version of the „arab spring“ were the following:
– The protesters were not armed and nobody armed them in the process either. There were few casualties among the riot police and the Bassij militia but not as a result of systematic guerilla like violence
– The protesters were not instigated by outside powers to fight against an autocratic regime that was tyrannizing and killing them on sectarian (or ethnic) grounds
– There is a persian saying „The knife has not reached the bone (yet)“ which basically means that despite many social injustices, reprisals, persecutions, economic inequalities and mismanagement and the governments constant interference in peoples private life…still daily life was very much on an acceptable level

In Syria, however, from early on there were deadly ambushes on army and police leaving to the deaths of dozens of security personell in the very first weeks. It is factually completely untrue that the protests were nothing but peaceful for months.
The protests were „contaminated“ quite early with anti-regime accusations and complaints on religious grounds. The state was accused of applying injustice and violence against its opponents because of the latters religious affiliation.
This was by and large utter nonsense but it was meant to serve a well-planned purpose, namely to defame a secular (although autocratic) government as sectarian. Not only there are many Sunnis in the highest political, economic and military ranks of Syrias elite, Bashar al Assad and his brother are married to Sunni women and their paternal grandmother was also Sunni. Now, one could check how many Saudi, Bahraini, Qatari or other „Gulf“ princes and „notables“ are married to Shia women…

The intention behind Syrias portrayal as an allegedly anti-Sunni regime was clearly to incite sectarian sentiment and play the majority card:
a) „Alawite“ Syria is between Sunni countries or border areas with high Sunni presence (Turkey, Jordan, Lebanon, western Iraq). This itself makes it totally inconceivable to accuse the Assad regime of having made the conflict sectarian
b) The western-friendly Arab petrodollar monarchies of the Persian Gulf are all ruled by Sunni Kings and Princes. They control the two most influential media networks of the Arab world: Al-Jazeera and l-Arabiyya
c) Hundreds of Millions of North African Arabs are Sunni allowing for a recruiting potential of tens of thousands of „Jihadists“ from among vast numbers of unemployed or socially weak youth. In fact it has turned out that the Jihad idea has attracted even central asian and european Sunnis

The question is whether there was a movement aiming at more reforms, freedoms, human rights and democracy that was hijacked quite quickly or whether this short-lived „secular“ revolution was on a too small scale to be considered a mass movement.
At any rate the power driving the Syrian insurgency is clearly militant Salafi islamism. The forces fighting are not even distantly moderate, academic or technocratic. Their motivation is establishing a (probably sectarian) religious state, not a civil democracy adhering to human rights:

„In fact, the only rebel factions still strong enough to resist and fight the regime on the latest fronts are the radical Islamists. The town of Azizeh, just outside the Marjeh area in the east, the strategically vital Sheikh Najjar industrial zone, the old city and Aleppo’s central prison are all defended by al-Qaeda’s affiliate, Jabhat al-Nusra, as well as Salafist militants Ahrar al-Sham, a member of the Islamic Front.“
http://www.al-monitor.com/pulse/originals/2014/02/barrel-bombing-syria-aleppo-rebels-regime-war.html

Of course, the US, UK and France along with their Arab „partners“, Israel and Turkey still want to stick to the now grotesque narrative that Syrias war is between a hated, russian/iranian-held, sectarian minority regime and the majority of „it´s“ reform demanding, peaceful, secular, moderate, pro western, democracy minded…people.
So, no matter how much it turns that the „bad guys“ are not only ISIS and JN, but also the other Salafi Islamists who
– are either openly hailing Al Qaeda and its principles
– or closely cooperating with JN (and sometimes even ISIS) as Al Qaedas Syrian branch:
http://www.thenational.ae/the-syrian-rebels-who-have-no-problem-fighting-alongside-al-qaeda
See also: http://100wordz.wordpress.com/2014/01/24/islamic-front-no-answer-for-syria-conflict/