Who knows, perhaps he couldn't cope mentally, or perhaps physically, and got sick of the training it took to run sub 20, which he did in 99 coming second to Mo Greene. He also was originally a 400m sprinter so he may have got sick of all the running off lactate. After 99 WC performance Portugal took him under their wing and now this amazing animal sprinter is fed and content. He gets hungry to perform big when he wants and once it rolls around to mid-trifecta period(OG), he means business, as was the case in 2004. The man has tremendous work capacity i'd say right up there with Justin Gatlin, Shawn Crawford, and Asafa Powell, as they were the only 4 men to make it thru the 8 rounds combined of the 100 and 200 Athens Olympics. He also had the work capacity to double up and win Eu Champs last year in the 2006 off year.
This man even brought the famed 400m arm-swing technique all the way down to the 100m and combined it with the leg-turnover of the powersprinter. A short arm-takeback descending from high up in the front(shoulder range) to barely past the hip, ala Michael Johnson and Jeremy Wariner combined with the smooth frequent strides of Asafa Powell. This is natural to him as he used to run the 400 meter, but he must have always had 100-200 speed, as he made the change to the more lucrative 100 event once he had to produce for his new country who put shelter over his head and kept him fed.
Here is his 10.3 100 opener from a few weeks back: