The only possible explanation was that the majority leader of the speaker of the house was not eligible .. or declined. I’m not sure you can decline, but I can’t see any other reason.
At the time of VP Agnews resignation congress was held by the democrats.
Speaker of the House—Carl Albert (D)
President Pro Tempore/Senate—James Eastland (D)
Therefore the next available Republican would have been Secretary of State William Rogers but he didn’t take or wasn’t offered the VP office.
I couldn’t find the House Minority Leader anywhere in the succession.
The only reason I can think for not following the Constitutionally mandated succession was that President Nixon didn’t want a democrat as VP, not a stretch at all.
I imagine Ford was a compromise both parties could live with.
I agree that no one with two semi-functional brain cells would let Paul Ryan (Weasel, Wisconsin) anywhere near the Resolute Desk. Not even for a photo!