Because he never was good enough to be called the best batsman in the world (middle order or anywhere else) ever. There is a tendency to call the best in the world purely based on current batting form thus we have seen the best batsman in the world tag being bandied around like ... I dont know what.
In about six years on CW I have seen the best batsman(opener, middle order or overall) in the world being used for Tendulkar, Lara, Ponting, Steve waugh, Hayden, Inzemam, Yoyo and Moyo, Pietersen, Sehwag, Gambhir, Dravid, Jayawardene, Kallis, Hussey and of course Chanderpaul.
It seems like everyone has an internal ranking system like the ICC or Suzuki or MRF ranking systems and it just, sub-conciously, works on the last known/remembered/recalled stats and decides on the number one ranked player. No one knows what the period of reference is of course. The shorter you make it the more often the best batsman(or bowler) in the world will keep changing.
There are enough ranking systems in the world to do this for us and they do it with a fixed period of reference and conscious and accurate working out of statistics.
I remember some one asking me here why I did not consider Chanderpaul too highly and gave me the batsman's stats over a period of time. I do not recall what I replied but the fact is that to say who has been scoring the most runs in the last three (or six or twelve) months (again ignoring attacks and conditions) is completely different, in my humble opinion than calling them the best in the world.
I am always reminded in such debates of Sachin's early career when he was quickly hailed as a great batsman when still in his teens. By the end of the series in South Africa in 1992-93, he had played 21 Tests and averaged 38.6 per innings Four months later he was twenty. In these four months he played four Tests at home against England (3) and Zimbabwe(1) and his averaged went on to 44.7.
In ODI's, by the time of his 20th birthday, he had scored 1520 runs at 32.34 with a top score of 84 - that's all. Not earth shattering by any stretch of imagination. Yet he was acclaimed the world over as the next great batsman from India after Sunil Gavaskar.
And he was - not because of his figures but inspite of them. You could see it when you saw him at the crease. This is what I dont see with players like Chanderpaul. Every time I see him score big rune I am amazed and wonder if the great bowlers of the past would have let him score so many. The fast bowlers from the West Indies itself in the 80's, the Indian spinners of the 70's, Imran, Lillee and Hadlee I would like to ask them what they really thought of Chanderpaul and his batting.
Mind you I am not saying he is a useless batsman. He is good and he is a very determined young man who has, inspite of his unorthodox stance and style scored very many runs at the top level but one has always wondered whether this could be sustained against the very best bowling for a longer period of time.
But the best in the world . . . . ???
PS : It is just an opinion and it is biased