Why did Spain agree to give Florida to the United States?

Florida had become a burden to Spain, which could not afford to send settlers or garrisons, so the Spanish government decided to cede the territory to the United States in exchange for settling the boundary dispute along the Sabine River in Spanish Texas.

Seminoles from Florida and run away Slaves attacking a settlement in Georgia ... 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.