Why is it so hard to form a habit? I know about the Cue-Habit-Reward cycle but still can’t make habits stick. Why?

It is about you if you want a habit to stick for example drink water every morning put a reminder for a week that you have to drink what at this time then when this week finishes you will already be sticked on that habit.

I say that it is hard because you are sometimes not used to doing things such as running. Sometimes goals don't stick because you may not have the motivation or just forget.

Consistency is everything it can take up to 33 days of a consistent routine to set a good habit
You are doing great just keep it up

Welland habits don't stick to anyone if they just get tired and quit so my suggestion is never be lazy to achieve a goal.