I'm sure someone will be able to give you a much more detailed answer, as unfortunately I'm travelling and don't have access to my notes. However, I can offer a little detail.

The notion of Islam not having a 'reformation' is a fairly common framework of armchair-analysis to explain the religiosity of the Middle East, and for western policy makers to understand why the region (and Islam in general, but by and large focused on the MENA region) has not followed the modernisation model - whereby European/enlightenment values (specifically secularism and religious moderation) are seen as the natural state to which all nations should progress. Now, these opinions are much more common in policy/politics than they are in academia where they have been heavily criticised.

In short, largely due to the impact of the 'Clash of civilisation' thesis and the popularity of this framework, analysits have sought to compare the development of Islam by contrasting it with the development of Europe. Central to European intellectual development was the reformation, and the lack of an obvious mirror event in Islam is held up as an important difference between the two 'civilisations.' However, this fundamentally misunderstands what both the reformation, and Islam, are about. The reformation was, largely (and forgive me here because my expertise is on the Middle East and not the reformation) about the emergence of movements that sought to counter the power and homogeneity of the Catholic Church. Protestant movements took issue with the monopoly that the Catholic hierarchy had over worship, and believed that it was not a necessary intermediary between the people and god. A central tenant of Protestantism was that individuals could connect with god and did not require the Catholic Church to act as an interlocutor.

Now, in Islam, there was never an institutional dominance of religion to match that of the Catholic Church. The path to becoming an imam has always been much more fluid, and there is no centralised body/homogenous power that mandates religious behaviour. The relationship between a Muslim and God has always been much more direct. Central to this is the fact that the Quran is the literal word of God, therefore anyone able to read the Quran is able to interpret the word of God. There wasn't the hierarchy of the European church which institutionalised levels of piousness and religious knowledge. Therefore, Islam has never required a reformatory movement in the same way.