This gets asked all the time and there are loads of threads about it
The general consensus boils down to a few things. Firstly, Edinburgh attracts a lot of Oxbridge rejects and very highly performing students. These students might be thinking that Edinburgh is like the Oxbridge of Scotland where you get personal tutorials and loads of help - they are left unsatisfied when they realise that isn't the case. They don't get personal tutorials, you have to put in effort and work for yourself.
Another reason is that the people that respond to student surveys may be more on the extreme end of the satisfaction spectrum - I heard that a lot of students in one department were irritated at the time it took to get a response about exam marking, so they did a kind of protest vote and marked the satisfaction down.
And then the last reason (I think this is the biggest one.) A lot of unis that are nowhere near as good as Edinburgh have high student satisfaction ratings. Unis like Robert Gordon, Napier, Caledonian, etc... The students at these universities will probably have a lot more time to just chill out and have fun. Why? Edinburgh is very academically focused, and students need to work really hard. They will see friends at Napier and other universities going out a lot more and will feel that they are under too much pressure. The fact is, a degree from these unis might not be as rigorous as Edinburgh. It's hard to get in to Edinburgh and the work is harder for a reason - the course is harder and employers know this.
To be honest, every Edinburgh student I have spoken to loves the university and is having a great time. This is why there is a problem with rankings and student satisfaction scores... Can you really get an impression of how people enjoy their uni from a score in a table?
The fact is, Edinburgh is one of the most prestigious universities in the world. It is consistently ranked highly internationally, has good employment and graduate prospects and attracts students from around the world. If the university was honestly that bad and people were really unsatisfied, then it would reflect in poor prospects, research output and things like that.