Some other reasons:
- Wealthy families (or tribes or religious sects) seek to establish places where they can be "free" of Earth's world government. (a la New England, Maryland, and Pennsylvania.)
- Earth's world government seeks to permanently exile particularly rich or admired people, without seeming to kill or imprison them. (a la "Australia".) If space travel is very cheap, it can even be used for ordinary criminals or political undesirables.
- It is a retirement plan for world leaders. Politicians can hand over power, and join a colonization mission, safe in the knowledge that nobody can haul them back to answer for their crimes (real or alleged). Plus, they can choose whom they will travel with.
- It is a matter of prestige for wealthy families (or tribes) to have a colony. (a la Maine.)
- A religious sect concludes that they can found a colony on their "heavenly" planet.
- A spectacularly valuable natural resource can be found in a colony system. Perhaps magnetic monopoles? Perhaps anti-matter?
- Spectacularly dangerous experiments can be done in such a colony, with little risk to Earth. For example, radical genetic engineering, anti-matter manufacture, or contacting the aliens.
- Some people are just impatient to "get started".
- It's a lottery. Enough people want to do it, and are willing to pay for a 1 in a million chance, to be able to pay for it. And the government is desperate for lottery profits.
- Pork barrel. All of the money spent (for space travel) is actually spent on Earth. Suppose some politically influential areas are dependent on xenon distillation, or huge ion thruster manufacturing, or expensive-but-light-weight reactor designs, or deuterium refining, or orbital launch systems, or waste-to-food-and-air converters. Suppose there is only a limited market for these products in the solar system. Suppose the only alternative to purchasing these products from these areas is to pay welfare to these areas. The colonization plan might be a politically viable way to shuffle money to these areas.
If economics are the only factor, it makes more sense to continue researching propulsion technologies, and building up Earth's industrial base. If the speed can be doubled in 25 years, that saves 150 years of travel time. Quadrupling speed in 50 years saves 225 years of travel time.