Perhaps it's because you're using the custom options object - you need to use the configurable options object to pass through configurable attributes using the "configurable_item_options" object.
{
"cartItem": {
"item_id": 0,
"sku": "string",
"qty": 0,
"name": "string",
"price": 0,
"product_type": "string",
"quote_id": "string",
"product_option": {
"extension_attributes": {
"configurable_item_options": [
{
"option_id": "string",
"option_value": 0,
"extension_attributes": {}
}
]
}
},
"extension_attributes": {}
}
}