Well, there's not only one reason, and it depends on from what country and where on the political spectrum is located the person you speak to, but to start with something here are some interesting facts that I've found out:

* It's got 448 articles, its preamble has 239 words, numbers that mean nothing until you compare them to the French constitution with 89 articles, or the US Constitution Preamble with 52 words.

* Alleged fascist bias, some articles promote social representation through organizations, workers unions, industrial organizations, etc,etc, unlike in Parliamentary Democracy where elected citizens represent citizen voters.

* It is said to have a neoliberal orientation, meaning it reduces the goverment activity role to security isues subordinating social welfare and the environment protection to the laws of market.

* One Central Bank for all countries, some people get nervous with the idea of having others printing money for them and deciding on the finances (inflation threat, wrong monetary decisions affect all, etc etc ).

* Bill of rights, not such: limitation of rights could be instrumented through laws, in other words, gives politicians the power to limit all kind of rights, essentials or not.

There many more and in different colors...