Well, that and the fact that there are newer strains out there, not covered by the vaccine, that have popped up in the past couple of decades.
All the DTaP and TDaP vaccines in the world won’t change the fact that you’re still not immune to these increasingly common variants.
http://wwwnc.cdc.gov/eid/article/15/8/08-1511_article.htm
It would seem that those strains emerged at about the same time as the acellular vaccines were introduced. The whole cell vaccine promoted longer lasting and probably more comprehensive immunity, since it would have provoked immune responses against all of the bacterial proteins, not just the toxoid.
I cannot look up any pertussis vaccines that are currently in development (because I am not at work), but my guess is that they are targeting multiple antigens to get around the problem of selecting for bacteria with increased toxoid production.