Why do students usually find dictation in English class very difficult?

Because , the English letters ,specially vowels, are pronounced in many ways. not as Arabic that each letter has one pronounciation. So that, they can`t write what they hear

that is not at time

if the english teacher is not taking to go for basic details regarding the topic and also give examples thats why students are interested to study english without any dictation

It's quite hard for anyone to learn a lot of words at the same time without attaching them to something to cut down the focus.

What is it ?

because students not interested in reading, 

Not all and not always. Mine often found it easy