If houses that were used to house tourists are no longer allowed to do so, why would they not become available for either rent or sale?

For rent because, depending on the laws, it can be really hard to get it back in case there is a tenant that do not pay or refuse to leave. In many italian cities there were many houses (they talk about 1/3 of the houses in Milano) that were empty because it was too dangerous to rent them (damages, missing payments, evictions which take years, people that refuse to leave even after the end of the contract). The same reasons make way harder to sell a rented house. So all (or most) of these house went to the short rent market (AirBnB and the likes).

For sale because the owner could keep it in case he need some extra money down the road or his son would need it some years from now or any other reason.

What else is there for the owners to do with them?

Nothing, which is better than to have to (eventually) fight to get the house back from a bad tenants, with all the time and money involved.

I see the point of what Barcellona (and other cities) want to do but the raise of short rents are a consequence, not the cause. True, renting on AirBnB make me more money than a normal rent contract but what people do not understand it that this system would have worked even if it would make me less money than a normal rent because 1) I would be sure to be paid, 2) I would be sure that the tenants would leave at the end of the rent, 3) where would be some sort of (partial) compensation in case of damages and 4) if I ever decide that I now need the house I just need to stop listing on the site and I have the house back.