Are you playing as the Korath?  They have the Unpopular (-2) trait that penalizes tourism severely.  Last time I played a race with that trait I had to build two or more tourism buildings on the same planet to overcome the penalty.

Quoting Unknown_Hero,

reply 2

Yes, I play as the Korath.
Retribution v3.96, standard races, all expansions and DLCs, no mods.
Ah, I see, what heresy!  
Unfortunately, this is once again a feature of the game that deserves to be redesigned properly.  
Stardock if you read us.

Quick fix. That would do the trick?
Unpopular (-1): -25% to planet Tourism Income
Unpopular (-2): -50% to planet Tourism Income

Thats how it used to be (-15%, -30%), got changed with 3.5. (I kept it as a % in my mod for this exact reason)

I like the tourism penalties as they are.  Without a significant penalty there's not a lot of reason not to take the unpopular trait so one can pick up other more important traits.  In other words, I think there should actually be a significant cost to taking that trait.  In fact, for several versions it was even higher and I wouldn't mind seeing it go back to that.  In that version if one took the maximum penalty he had to build even more tourist buildings on one planet.  I usually built every such building on one planet to maximize the income because my custom race has the unpopular trait.