- In some games you are unable to play on the harder difficulties before complete it on a lower difficulty. Why?
- Achievements are popular. In some games you do not get an achievement for completing the game on the hardest difficulty (e.g. Bethesda RPG's) but for just about everything else, Why?
First of all: Why someone made a specific design decision is pure speculation. We can not read the designer's mind. Also, do not assume that every design decision you see in a popular game is necessarily a good design decision. You can find a couple of design flaws in every game. The best games are those which compensate for them by having plenty of greatness in other regards.
Regarding unlocking a harder difficulty after the player completed the game on a lower one, possible motivations can be:
- It prevents frustration. The hardest difficulties are usually balanced for players who already completed the game, already mastered all the game mechanics and are now looking for a proper challenge of their mastery. The designers believe that a new player will be overburdened by this challenge and will just be frustrated by the experience.
- It is a cheap way to increase long-term motivation. The player won't really feel like they completed the game before they completed it twice: Once on normal difficulty and once on the now available hardest difficulty.
- It is a reward for completing the game. When the player gets something new to do after completing a goal, that's a form of positive reinforcement.
Regarding achievements for completing the game on different difficulty levels: Bethesda games usually allow the player to change the difficulty level at any time. They even encourage that: When the player dies a lot while on a higher difficulty, they will often get a loading screen hint suggesting to try a lower difficulty. But when you want the player to continually adjust the difficulty to their liking, you can't really give a reward anymore for completing the game on a specific difficulty. Not just because that would go against your design decision, but also because it now gets hard to measure how much of the run was played on what difficulty.