Because it has a high blood supply. One of the main route of metastasis is bllod born metastasis,, so organs rich in blood supply are at higher risk of being the host metastatic cells, like Bone, Brain, Liver & Lung (BB & LL).
Some cancers are more prone to metastasis to the bones than others. Blood supply is an issue in transporting the cells, but the chemical environment of the bones also plays a part. See the references below:
Why Most Of The Cancer Types Make Metastasis In Bone?
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