Washington was universally highly regarded. There was indeed something of a personality cult around him in the years after the War. He was not without flaws, no question- his lust for land and pursuit of property make him a good candidate for America's First Developer; but many others were as greedy. And he was quite ambitious. But he tried very hard to maintain an air of gravitas and impartiality that only slipped occasionally ( like in his over-reaction to the "Conway cabal"). He had also not only kept an army in the field for the War but dealt diplomatically and effectively with the Continental Congress for the duration. Many of the delegates to the 1787 Convention had been in that Congress, had real experience of that, and knew of Washington's talents first hand.

His stance of being above parties and factions was as important as his abilities. The thirteen states were forming a much tighter coalition than under the Articles of Confederation, and there was strong Anti-Federalist opinion that too much power was being given to an executive, that this more powerful national government would become despotic- that large states would tyrannize the smaller ones, large factions would subjugate small ones. Washington's studied neutrality made him much more generally acceptable than others. John Adams was immensely ambitious, but even he had to admit of being a second choice to Washington. Adams was a Northerner, and there was a general cleavage between the interests of northern states, like his Massachusetts, and Southern ones. It is not by accident that for the first decades of the Republic the Presidency was held by either someone from Virginia or from Massachusetts.

Washington also didn't stay there in the Presidency. I am not sure how early and how widely he let it be known that he was not going to stay in power, but the Society of the Cincinnati ( the fraternal organization of former Revolutionary War officers) were named for the legendary Roman general who left his farm to lead the army to victory and then went back to his farm. Washington was known as the American Cincinnatus. He served only two terms as President, and was happy to leave- the pinnacle of ambition for every boy in Virginia was to be a wealthy planter, and he wanted to go home and be that. But he could have just as easily continued, and I think it's commonly accepted that setting this precedent was one of the most important things he accomplished.