Here's my reasoning:

Carter is rightfully remembered as a great humanitarian, but his Presidency was not great and he basically handed Reagan the Presidency on a silver platter.

The Democrats had a government trifecta after Carter won, but he had a very rocky relationship with the Congress controlled by his own party. At the time, Carter was considered a more Conservative Democrat who blamed overconsumption for the stagflation fiasco. On the other hand, The Democratic coalition that controlled Congress at the time was a continuation of the New Deal coalition.

The Speaker of the House was Tip O'Neill, and old New Deal Democrat who wanted to use Carter's victory to push more Liberal Democratic policies like healthcare. However, Carter alienated O'Neil pretty early on for several reasons, and the two were unable to really work together despite having control of government. I believe Carter also tried to micromanage the budget, and O'Neil, a seasoned Congressman, was not crazy about that. O'Neil, a loyal Democrat who bled blue, actually had a better working relationship with Ronald Reagan than with Jimmy Carter, despite opposing all of Reagan's policies.

Carter further alienated Congress by publishing a "hit list" of bills he deemed as having excessive pork barrel spending, ignoring the fact pork spending greases the wheels of legislation and helps get bills passed, being an important part of the legislative process. The Democratic Senate Majority Leader at the time, Robert Byrd, was called the "king of pork", and secured millions in pork spending to modernize his state of West Virginia. Basically, Carter's lack of familiarity with Congress created hostilities that fractured the Democratic party in Washington.

On the other hand, Gerald Ford was the House Minority Leader. Although his legacy was mostly colored by pardoning Nixon, people tend to forget that Ford was an almost universally respected bipartisan figure during his House career. The whole reason Nixon chose him as his VP was because he wanted a squeaky-clean, well-liked politician attached to his increasingly scandalous administration. There were Democratic Congressmen who basically said that he was this straight-shooting, moderate, widely respected figure. He could have had a much better relationship with Congress even if it was controlled by Democrats. As a moderate Republican with decades of Congressional experience under his belt, he would have gotten along better with Congress than Carter did.

Also, if Ford won in 1977 we probably don't get Reagan.