Why don't half of us have situs inversus?
In the Wikipedia article linked to in the question, one will note that there are several medical conditions associated with situs inversus, including congenital heart disease, primary ciliary dyskinesia, and Kartagener Syndrome. These can easily lead to reduced fitness for reproduction, especially PCD/Kartagener Syndrome, as one effect is male infertility due to lack of functional cilia on sperm. 

Therefore, there likely is a negative pressure on the alleles responsible for this condition, explaining why it is so rare.