The Court [in Merrell Dow] saw the missing cause of action not as a missing federal door key, always required, but as a missing welcome mat, required in the circumstances, when exercising federal jurisdiction over a state misbranding action would have attracted a horde of original filings and removal cases raising other state claims with embedded federal issues....I don't know if I like the whole welcome mat/door key addition to the usual door metaphor in jurisdiction, since courthouse doors never have welcome mats and litigants don't need keys to get in the courthouse door.
Expressing concern over the "increased volume of federal litigation," and noting the importance of adhering to "legislative intent," Merrell Dow thought it improbable that the Congress, having made no provision for a federal cause of action, would have meant to welcome any state-law tort case implicating federal law "solely because the violation of the federal statute is said to [create] a rebuttable presumption [of negligence] ... under state law." 478 U. S., at 811-812 (internal quotation marks omitted). In this situation, no welcome mat meant keep out.
Maybe try something with a metal detector next time.