There are actually several reasons, and i will get into them:
First and foremost, the WI wasnt composed of 4 greats. Chanderpaul was extremely new to international cricket during the period which you allude to, and theres no way anyone can just walk into international cricket and become great. It involves years of hard work and improvements to your game. Even now i dont consider Chanderpaul to be great, hes about as great as Azharuddin was(minus the match fixing) because his record is inflated at home and quite ordinary abroad.
Also not long after he made his debut, his short comings as a batsman were clearly exposed- he didnt really have an off side game, and most bowlers realised that considering how dependent he was on leg side shots, all they had to do was keep on bowling on the off side and he wouldnt last very long and this is emphasised by his slump from 97 onwards until 2000(once again coinciding with the period you refer to). Of course teams like India quite clearly seemed oblivious to that, and on their tour to the WI in 96/97 decided that they would be better off by bowling onto his pads and around middle stump and as a result he had his most prolific series of his entire career uptil that point. He has of course since strengthened his offside game and his record as a result has improved dramatically.
Another point of note is that Lara from 96-01 was hardly deserving of the word 'great', in fact the SL tour of 2001 was supposed to have been the rebirth of his career. His average fell from 60 odd in 95 to less than 50 by 2000 or so, and while he still had his moments in the sun it was clear that for whatever reasons his career had gone straight down the barrel.
Also the support players, Hooper was quite good, Richardson played his last test in 95 and even he was past his prime by then, Adams was distinctly ordinary post 96, Bishop barely ever played and on the rare occasions that he did play was not even half the bowler he was when he first came onto the scene and anyone who thinks that Gayle is or has ever been more than acceptable in tests should have their head examined( in fact i doubt hed make too many other test sides other than the WI at the moment). This meant that for the large part WI was represented by Philo Wallace, Stuart Williams, Sherwin Campbell, Nixon Mclean, Mervyn Dillon, a very poor Jimmy Adams, Rawl Lewis, Adrian Griffith and miserable Gayle and Hinds all of whom were either distinctly ordinary or complete crap.