Because of the conflict of IDs for the filter. In case of the search the plugin “switches” database to look within the attribute values.
In case of filters the values are meaningless, the posts are connected to the attributes via their IDs. It’s sort of a N to N type of connection between unique keys (IDs). Because these keys (IDs) are only unique for each blog then there is currently no way to print the filters and use them because of this conflict.
In theory there is probably a way to get around it by somehow marking/transforming the terms with the site IDs, but currently there is no modification I can suggest to do that.