There is only a limited space on the 'close as off-topic' menu for other sites, but there's frequently another site not in the list that might be suitable for the question. For lack of any other option, these get a comment (if they're lucky) and closed 'off-topic' with no suggested alternative site.
Given the number of possible sites, should there be a write-in option for the alternative site? The validation could legitimately limit options to 'the trilogy' (since ServerFault isn't listed) plus valid top-level domains of Stack Exchange.
Related questions include:
- More flexible off-topic migration
- Should off-topic migration close votes be differentiated from others
but both pre-date the review queues as they now exist.