This book provides many good answers as to why there is democracy in the Middle East. Whether any of the answers are the right answer is a completely separate question. As stated in the conclusion, the four key factors prohibiting democratic transition have been identified as:
1)international (Western) support for regimes' coercive apparatuses and also for the authoritarian regimes in general
2)hydrocarbon (a political science term for OIL) and aid-based rents creating the rentier state (reduces accountability of regimes)
3)the level of state intervention in the economy
4)the presence of powerful, undemocratic (often Islamic) opposition groups instilling fear on behalf of many parties in what a democratic transition could engender - i.e. another Islamic republic
I would also include the state of the opposition, which is slightly included under #1, and is obviously not unique to the MENA region, but a divided opposition, whether it be between legal and illegal groups or between secular and religious, cannot form a legitimate challenge to the state.
A good read that covers many of the basics about the region's "democracy deficit."