Choroid. Because there are a lot of blood vessels in the choroid. Cancer can spread through lymphatic organ or blood. Cancer can spread to the eye hematogenously/via blood and choroid would be the one that is affected the most.
Blood supply. Metastatic tumors like to take up residence in richly vascularized tissue to give them a good blood supply. The choroid is a rich vascular network supplying the overlying retina and metastatic tumors gravitate towards this tissue.
Anatomy. While the choroid is not an extremely common site for metastasis, it is not rare. Why it happens is likely a combination of factors which include the specific nature of the particular cancer cells and the anatomy of the complex vessels of the choroid.