As to your question, state-law claims can, at the option of the plaintiff, sometimes be filed in federal court, depending on the citizenship of the parties (generally, if the plaintiff and the defendant are from different states, federal court is available). If he has the option, whether a plaintiff decides to sue in state or federal court depends on a number of tactical factors. I don't want to get to technical, but to give one example, there may be different procedural rules in state and federal court as to when an appeal is available. Or, the lawyer may just be more comfortable in state or federal court.

Jury pools may be a factor too. If a plaintiff can bring a case in state court in The Bronx in NYC, he may choose to go there rather than to the corresponding federal court (which is either in White Plains or Manhattan), on the theory that Bronx juries are notoriously generous. In other situations, the roles may be reversed, and it may be in the plaintiff's interest to sue in federal court. There was, for example, a federal judge in Alton, Illinois about 20 years ago who just hated corporations. Every plaintiff in the area would if possible sue in federal rather than state court because, basically, they couldn't lose.

Finally, just to make it clear, the important role played by state courts and state law is not limited to product liablity. Virtually all states, for example, have various consumer protection laws, often with treble damage remedies, attorneys' fees, etc. Even in employment and housing discrimination, which you probably think of as largely a federal matter, all states that I know of have such laws, and often they are more protective of individual rights than federal statutes, and plaintiffs often choose to sue in state court under state law.