Because they are different games and the developers can do different things Dark souls has built its brand on being hard and there have definitely been a lot of people who complain about there not being an easy mode.

Pokémon never established the fact that it's supposed to be easy just that it's meant to be playable by all age groups which tends to entail atleast a modicum or option of ease.

If they want to have a hard mode or not is up to them, but regardless players are going to complain about their decision. Each company can make any decisions they want but you can never make everyone happy