Because you cant rule too many town, King not allow that
There are a few possible ways to get around this before the expansion is released.
1) Become a Rebel Warlord. To do this, you'll need to either renounce your oath to your king (talk to him for the option), or start a new game and not join a kingdom to begin with. In order to "declare war" on a faction and besiege their cities and castles, you'll first need to attack a caravan or Lord party that belongs to that kingdom. Once you own land as a rebel, it's possible that other kingdoms will declare war on you, and it'll be impossible to sue for peace unless you join another kingdom that you're not at war with.
As I implied with the previous sentence, it's possible to keep this Warlord status as a temporary measure. Attack and capture whatever lands you want the most, and then join one of the other kingdoms. Your fiefs will remain in your possession and join that faction just as you do (ie, if you join the Vaegirs your settlements will turn green, but still belong to you). If you later renounce your oath to that faction, I
think you'll lose your lands, but I'm not 100% sure.
2) Start a claimant quest. When you sign on with one of them and start the corresponding rebellion, you become a special type of Marshall. You will have the standard Marshall options (like giving orders to Lords that join you), along with the ability to choose who gets control of a particular fief that your rebellion just captured. Be aware that Lords who voluntarily join you will keep their previous lands, and will get upset if you award a lot of land to yourself, and give them little or none (the Claimants can get angry over this as well).