When we first got to Rohan we had a level cap and time to level the steed and trait all trees to cap if we chose. Perhaps without an enforced halt people are taking on things with less advanced mounts? Also you only get mount experience by killing whilst mounted so if you are doing everything you can on foot you won't be advancing the mount. Making the quests that make you mount up even tougher. The consequence of your decision and no access to capstone traits that make them easier.