Well, Final Fantasy X is a mainstream RPG that's more challenging than most for the average player who follows normal game progression in the game. Additionally, there are hidden bosses that are difficult to beat even if the characters have maxed stats.
Of course, "difficult" in a turn-based game like FF usually means a long battle where the player is constantly under pressure to heal and keep his/her party alive, while whittleing away the enemy's health. Once you know exactly what to do, then it all just takes time. (The exception is the occasional cheap boss who deal unpreventable instant death and requires luck to beat).
Regardless, the game can seem difficult to most players, even experienced ones.
People buy games for many reasons, but ultimately the casual gamer buys a game for entertainment. Now, entertainment is relative to the player; some may think a challenging game is fun, while others think an immersive game is fun.
Those who favor a somewhat engaging and interactive role-playing experience, will generally favor KOTOR 2 more. Remember, role-playing isn't all about battles and fighting.