People got used to simple games
With the rise of the mobile gaming industry many people got used to simple and casual games. RTS games in comparison to those games are much more complex and so many people have trouble adapting to them.
Complex learning curve
Many RTS games have a complex learning curve where a person can hardly master the game in a short period of time. Many people find RTS games much harder compared to the games with short learning curves.
Too much information to remember
RTS games usually introduce so much information to the player such as the different unit types, the power of every unit, its strengths and weaknesses. Because learning the game requires knowing too much information, the game can be complex to many people.
A person has to multitask
For a person to win at an RTS game he must multitask by doing so many things at the same time. This can include overseeing the base while trying to attack others.
Stress makes the game harder
Since RTS games are real time a player can experience lots of stress as he has to race with time to win or to be able to defend his base. Stress can make the game harder. (See Why Do Gamers Get Very Angry?)
Time limitations make them harder
Since RTS games are real time a player has to build his base and army in a timely manner in order not to lose. Being good in an RTS game isn’t enough if a player can’t get things done fast.
Mistakes build up
In an RTS game any mistake made can’t be erased as the game is real time. A player has to do his best to do few mistakes in order to win.
Lots of micromanagement needed
A typical RTS game requires a lot of micromanagement as the player has to keep an eye on so many different buildings, structures and units. This makes RTS games harder to play.
Newer generations can find them harder
As RTS games became less popular some newer generations never had the chance to invest a lot of time in them and as a result those people might find the games harder to learn compared to the easier ones they got used to. (See Why frustration is an important part of game design)
They require a lot of experience
In order for a person to master an RTS game he needs a lot of playing experience. Less experienced players can easily lose to more experienced ones.
The player has to plan and think ahead
Unlike many other games where a player reacts instantly according to what’s happening, RTS games require lots of planning in advance in order for the player to win.
Why RTS games are so hard?
Why Real-Time Strategy (RTS) Games are so hard? Why do strategy Games seem too hard for me?