The Federalist Party, led by Alexander Hamilton, was in favor of ratifying the newly written and modified Constitution. The United States was free of British control after the American Revolution. Their first official attempt on having a formal government was a document called Articles of Confederation. Though that ultimately failed due to no taxation, no faith and interest from the government officials and gave too much power to the states. The newly formed Constitution proposed a strong central government and unite the whole colonies together. In that way, they could raise their own money through taxes, fund and direct a national army and deal with territory that was not part of the current states. The weak and unfavorable points of the Articles of Confederation appointed.
There should also be a consideration on facing a threat of invasion by France or England. The Constitution would be able to keep all the states unites instead of frustratedly trying to fight them off again. With the Constitution ratified and in place, the national government would have a better standing in the world. Also the power, in the country will be distributed by Congress, with Senators and House of Representatives. The stated reasons above