Why was the mission built in the area of San Francisco?

Because there are sinners everywhere in need of salvation even in the 1700s...and especially there in the 21st century. That and because this mission is part of a chain of missions built along the West coast beginning in Mexico in Baja California. The Spanish missions in California comprise a series of 21 religious outposts or missions established between 1769 and 1833 in today's U.S. State of California. Founded by Catholic priests of the Franciscan order to evangelize the Native Americans, the missions led to the creation of the New Spainprovince of Alta California and were part of the expansion of the Spanish Empire into the most northern and western parts of Spanish North America.

It was easy to get to though water