The Syncing of the Smart Collections to LR Mobile is not possible, however Collections can be synced.
As it is the Collections that gets synced with the LR Mobile, that is the reason it is not listed under Publish Services.
Also it is a restriction that you can sync Collections from only catalog the LR Mobile.
Please let us know if it helps.
No, Smart Collections cannot be used as the mobile sync collection. You could drag the results of the smart collection into a regular collection for mobile syncing, but that is not the same as syncing a smart collection, as you are hoping.
You are correct in that mobile syncing is done via one collection in one catalog.
As to the why's, Adobe would need to answer those; and they are not going to do that.
Not an ideal solution, but a workaround..... Use at least one smart collection to monitor the synced collections.
For example, let's say that ideally I want to sync my 3 star images of Florence, Rome, and Naples. I am going to need three separate collections and populate them by finding those images manually. So create a smart collection of 3 star images where the city is one of those three and collection name doesn't include Florence, Rome or Naples (remember the Alt/Option trick lets you nest smart collection criteria). When this smart collection has zero items, you know your dumb collections are correctly populated.
The ability to sync smart collections is the most requested feature on Adobe's official feedback forum. Please add your vote and opinion to this topic: Lightroom: Ability to sync Smart Collections to Lightroom Mobile / lightroom.adobe.com. That will (in a small way) make it more likely that Adobe will prioritize its implementation.