France was ready to sell Louisiana because a slave revolt in Saint-Domingue, modern-day Haiti and the Dominican Republic, wrecked Napoleon's plans of establishing a French empire in the Americas.
Fearing that such an empire would be more trouble than it was worth and with war with Britain seemingly imminent, Napoleon made the decision to sell the Louisiana territory for the sum of $15 million. At about four cents an acre, this was an absolute bargain.
Jefferson viewed the Louisiana Purchase as a great opportunity. Not only would it make the United States a richer, larger, and more powerful country, it would also allow more Americans to follow the Jeffersonian ideal of becoming yeoman farmers.
To Jefferson and his supporters, this would provide a firm basis for the kind of republic he wanted to build, as they believed that independent landowners provided a bedrock of political stability for a nation that was still relatively young.
They also believed that yeoman farmers were the natural supporters of limited government, a key Jeffersonian ideal. In that sense, they would, it was hoped, provide a counterweight to the East-Coast business and banking establishment, which tended to favor a strong central government and was strongly opposed to Jefferson's presidency.
The Louisiana Purchase, then, was envisaged by Jefferson as a measure that would greatly benefit his party at the expense of his Federalist opponents, who had little support among rural interests.