Tourism dollars, or euros in this case.

It started to lean during construction. From wikipedia;

The tower began to lean during construction in the 12th century, due to soft ground which could not properly support the structure's weight. It worsened through the completion of construction in the 14th century. By 1990, the tilt had reached 5.5 degrees.[3][4][5] The structure was stabilized by remedial work between 1993 and 2001, which reduced the tilt to 3.97 degrees.[6]

How do you know it's constipated?

Tilted.

Don't look up an artist named MC Escher then