George Washington's selection was not a foregone conclusion but there really were no other candidates.

Washington was a member of a leading Virginia family and one of the wealthiest men in the country at the time with a net worth of over 500 million dollars when adjusted to today's dollars.

This meant that he could finance a military campaign, which he pledged to do at a special March 1775 meeting of the Virginia Assembly:

Rather than submit to the present condition of things, I will raise one thousand men, subsist them at my own expense, and march myself at their head to the relief of Boston.

This sentiment is expressed in HBO's John Adams but in a different context.

As you can see in the clip, Washington wore an elaborate and custom designed military uniform every day to the Convention. At a time when it was considered unseemly to campaign openly for a job, this was a subtle reminder to everyone present that Washington had substantially more military service than any other colonial leader, even if he never commanded a large army in the field.

At the time much of the fighting was happening in the north, Washington was a Virginian and as such he brought the south into what was perceived by many as a northern cause.

Washington basically had everything working in his favor and without a real alternative candidate.