Keta,
It's all about politics. The Interior Department used to house both the U.S. Bureau of Sport Fisheries and Wildlife, i.e., the U.S. Fish and Wildlife Service, and the U.S. Bureau of Commercial Fisheries. The story I've heard is that Nixon got pissed at his first Secretary of the Interior, Rogers Morton. Part of the shakeup included moving the Bureau of Commercial Fisheries over to the Department of Commerce and naming it the National Marine Fisheries Service, where it sits along with the Department of Oceans and others.
Their jurisdictions overlap mainly with anadromous fish, which are both freshwater and marine fish species. It's still weird though, USFWS has sea otters, but NMFS has seals, sea lions, and whales.
Politics and logic are not necessarily a good fit.
Sg
That's the right answer, except.......
Nixon's first Interior Secretary was Wally Hickel, from Alaska. Nixon fired Wally because of his views on Vietnam. Wally was then replaced by Rogers Morton as Interior Sec.
A better question would be: " Why can't we combine these two Federal agencies?" The current Prez mentioned this a couple years back, in a State of the Union address. He included a quote on smoked salmon, as I recall. There was lots of activity for a couple months afterwards, and then complete silence. Have not heard anything since.