It's easier for me to understand that he could fail in the presidential primaries of 1968, with smaller experience, weaker surname and (imho) stronger opponent. But why did it happen in 1976?
According to Wikipedia entry and it's cited sources, there were many arguments against Ford that were used during the campaign. First of all, Ford was in the administration of Richard Nixon, what already speaks for itself. He even grant a presidential pardon to Richard Nixon for his role in the Watergate affair. Also his negotiations about secession of Panama Canal were more in the Democrats' style, and were later continued by Carter, to which Ford finally lost the election. There were also other accusations, like the current situation in Vietnam or signing of Helsinki Accords, what at that time also sounds to me more like Democrats than Republicans.
From my transatlantic perspective and lack of deeper knowledge about America in the 70s, it looks like it should finish with a quick K.O.
I also wonder if the fact that Reagan got divorced in 1949 (and was later elected as the first divorced president of the USA), could be one of the reasons for that. I mean, he was a candidate of the conservative wing of Republicans, which I think could care about such fact. Did Ford try to play this card at all?
Or was it the fact that Reagan was in the Democratic Party before the WW2? What were the main reasons of his fail?