One of the reasons haemophilia appears to be more treatable by gene therapy (than other diseases) is because the cells that should be producing the clotting factors are found in the liver. When virus carrying the gene therapy enters the blood it is collected by the liver's filtration mechanisms, conveniently aggregating the gene therapy in the tissue that the gene therapy is targeting.

It is hard to target gene therapy toward specific cells/tissues but the liver makes targeting haemophilia with gene therapy easier.