Thanks mates for your expressing your thoughts! Gulshanji has rightly explained that spoken English is a differnet ball game altogether.Writing articles can help you in strengthening your general English but not 'spoken' part! The difficulties arise mainly due to the varying demands of it. Just imagine a situation when you meet a pal after a long time and the kinds of issues which might be taken up for chatting - from boss's rudeness,kid's recent illness to Dhoni's sudden loss of form! That too out of turns in broken sentences!!!And in an article you are writing on a particular topic with enough time to take care of your thoughts and grammar!!!
When we have to face anyone and have to speak fluent English,the thoughts which should come in mind,doesn't comes to our tongue immediately. This can only,when we had lot of Experience of Spoken English of Different Language.
I would say learning English is different thing and applying English in yourself is different thing. Until you apply that learning in yourself,Its difficult to Speak Fluent.
When we write,we think and then write but Speaking infront of Someone,Our thoughts gets less time to think and speak fluent.
Answer is lack of self confidence and practice. But if he tries himself, then it will not be a difficult task to speak any language fluently. Improvemence can be brought bit by bit, not all of a sudden.