Question

Why did spain so readily agree to sell florida to the united states after years of colonizing it?

Answer 1

Florida had become a burden to Spain, which could not afford to send settlers or garrisons . They decided to  cede the territory to the U.S. through the Adams-Onis  Treaty in exchange for settling the boundary dispute along the Sabine River in Spanish Texas

Answer 2

They realized that they could not keep the United States from talking over the Florida territory so in 1819 Spain agreed to sell Florida to the United States. The Adams-Onis Treaty was approved by Spain and the United States in 1821.