Because of trial lawyers who present sympathetic cases, whether caused by the vax or no, to juries, who are happy to punish vaccine manufacturers, who are portrayed as the big bad wolves. They can be, we have learned in spades recently, and perhaps they are, nowadays, but they also make MMR, the subject of this piece. There were many lawsuits over issues like thimerosal and autism, and reasonable people, we see in the piece and the comments, can disagree on the evidence. But the upshot was that the manufacturers threatened to simply stop making any vaccines, so (in 1986, maybe?) Congress passed the bill protecting them, but setting up the fund that was meant to pay for childhood vaccine injuries substantiated through [some process, the details of which I don't know].