"Beating an incumbent is hard: Ronald Reagan, George W. Bush, Bill Clinton and Barack Obama won their reelections not because they were great presidents but because they were incumbents."
infers to me he feels Reagan and the others were not great Presidents in the opinion of the writer. Reagan was a great President, perhaps the greatest modern day President, and that is why he won re-election. The writer is too lazy to make the distinction and throws Reagan into the same group as the other mediocre at best list of Presidents then I am to lazy to keep reading.
No it doesn’t. That’s a very common sentence structure and it means... well it means exactly what it says. The thing in the back clause is not being questioned as existing, simply being pointed out as not the cause of the event. It’s like a team with a great QB dominating the run game “they did not win because they have a great QB”. Doesn’t say the QB isn’t great, simply not the cause.
As for his list they’re 3 incumbents that won in a row. And they all won NOT because of their quality as presidents, but because the mushy middle loves incumbents. These are facts.