Call me a cynic, but I think its very obviously a financial issue. As this article explained, the use of thimerisol allows for a multi-dose vial which is cheaper than a single dose vial. That's the bottom line. There is no liability because of the 1986 law that created the NVICP (which paid out over a
billion dollars in damages in taxpayer, not pharma co. money, in it's first 10 years).
DTP was cheaper to produce (by $9 a dose) than DTaP, and so its production/use was delayed in this country for decades despite the fact that the whole cell version contained toxins which caused brain damage and/or death in significant numbers. Same with OPV vs. IPV. OPV was the only cause of polio in this country for almost 20 years - but IPV costs nearly twice as much to produce.
Despite the costs, thimerisol has been [mostly/kinda/sorta] removed from vaccines, DTaP is the standard now, as is IPV... all with the assurance that there was nothing really wrong with the old versions. Yeah. Coughbullshitcough.