There have been many (many) questions around here about the how, why & even when will man kind become a multi planet society... this is not one of them.
This question in fact goes the other way around, why would humanity fail to become a multi planet society?
Assume for a minute that all of humanity (yes every single one of us, it may be unrealistic but this is world building after all) is all of the sudden willing to put aside religion, consumerism, pride, ethnicity, politics & all the other silly reasons that divide us and we all agree that settling another planet (at least one) is super important and are willing to sacrifice a lot of comforts to get it (let's say around 15% of the total world GDP as well as mass agreements that needed resources and laws are passed to help make the dream a reality).
Starting tomorrow morning (literally tomorrow morning is the starting point of time) every single man & woman alive will work to create a self sustaining colony off world with the undivided attention, desire & resources of the entire world working together to reach that goal before 250 years pass (why 250? because)
Seems great right? now the question is what is a possible realistic reason (which isn't a natural disaster sending us back to the stone age or wiping us out) that will make us fail that goal?