It's because it has not been imposed or instigated from outside, and has been launched ostensibly to restore the hopes of the original revolution, rather than destroy it. This is what many neocons still don't seem to understand, which is presumably why so many seem to be outright hoping for the coup to succeed:
I think it’s critical to understand the context in which this threat arises. The protesters are not calling for an end to the concept of an Islamic state far from it, in fact. Just because elements of the opposition are seen as friendlier to the West, it doesn’t translate into a monolithic “Westernizing” force. And that is why it is potentially so dangerous to the stability of the status quo.
The leading faces speaking out Mousavi, Rafsanjani, Montazeri are figures who were among Khomeini’s inner circle in 1979.
Montazeri, a Grand Ayatollah who was once seen as the successor to Khomeini, is of particular note. Think of it: a man who was once considered a probable successor to the founder of the Islamic Republic has now come out and said, in no uncertain terms, that the protesters are right and the government (including the clerical leadership) is wrong. No longer are we just questioning the legitimacy of Ahmadinejad’s presidency rather, Khamenei himself and the institution he represents is called into question for abetting it. The most serious threats, it seems, are always from within and this is just one example of the fault lines developing within the clerical leadership that will reverberate for years to come regardless of how the current crisis is resolved. Once the “crisis of legitimacy” genie is out of the bottle, it’s impossible to get back in. People will always remember and internalize these events, coloring how they see every subsequent action by the government (much in the way the U.S. assisted coup in 1953 still hangs over U.S.-Iran relations)."