Why is it so hard to read on a computer screen?

According to a study People build mental maps while reading which include the location of the information they are reading. In a book it's easy to recall where a piece of information was but on an internet article or PDF it's much harder to do the same and that's why reading becomes harder.

When a person holds a book he can easily feel it's weight and know exactly how far he went through the book. This makes the process of building the mental map , which includes determining the location of information, much easier.

Because people on the internet usually jump from one action to another the mental state they experience when they read from a screen can prevent them from staying focused for long. 

The varying fonts on computer screens can make it hard for some people to read some material that are written with small fonts.

Some people experience eye strain when they look at computer screens for long periods of time. Those people usually find it hard to read directly from a screen.

In one study it was claimed that comprehension is reduced when a person reads from the screen. As comprehension becomes harder more mental effort is required and thus reading from a screen can be harder. (See Why some people find it hard to read a book?)

According to NNgroup reading from a computer screen is 25 percent slower and so more effort need to be exerted to read the same text from the screen.

The availability of so many options on the web puts the reader in a non focused mental state as he always keeps assessing the material he is reading to find out whether he should continue reading it or whether he should leave. (See Study summary: How much users read on the web?)

The lighting settings is different on computer screen than when a person reads from paper. This can make some people find reading uncomfortable especially if the lighting conditions are not properly adjusted.

The image we see on a computer screen is constantly refreshing. This can be tiring for some people.