Civs have war maps in RFC-type mods, which guides them where they try to expand.
These values are independent from the owner of the territories/cities (apart from a couple special cases).
Constantinople is very high priority for quite a few civs, including the Ottomans.
In the majority of games I saw they do get Constantinople, especially if it's independent.
The only expection is if Byzantium remain very strong on the Ottoman spawn.
That should only happen every once in a while, it's a problem if it happens too often.
But that's a different issue, not related to the Ottoman's willingness to go for Constantinople.