William Walker and his followers didn't want to buy Nicaragua; they expected to conquer it. Not everyone believed it was possible, but enough Southern men thought it probable that it was a topic of popular discussion. He also had investors.
Consider the alternatives: conceding most reasonable Southern men were Unionist before events of 1859-60, what were the options for remaining in the Union and maintaining a semblance of political parity? The House of Representives had long since been lost, based as it was on population. The Senate, designed as body to represent all states equally, was slipping away. Southerners hoped to maintain political power through admission of more slave states by any means possible, and by means of the Democratic Party. The South might be safe if Democrats voted along party lines to defeat the schemes of the Republicans. Beyond a president and supreme court members friendly to slave interests, what did the future hold?