Hard to call any of the various reasons the biggest. Space travel is an evolving discipline requiring vast amounts of money, step-by-step engineering progress, learning through practical experience which takes time, political commitment, and constantly changing public opinion. In theory we already know how to do it but in practice we don’t know all the challenges we’ll run into, and therefore have not solved yet. The long-term ROI is unquestionably huge but unknown. For example, platinum is currently worth almost $1000 USD/oz, while aluminum is about 8 cents. If platinum became as available as aluminum this would radically change, and we would discover new uses for platinum that haven’t been imagined yet because it’s so expensive - nobody would think of making pie pans or window frames out of it, but physically it might be far superior. Its properties haven’t been explored nearly as fully as the properties of aluminum, but they would be, and nobody knows the result. Maybe there’s an easy way to do antigravity using platinum. Whatever - the point is we don’t know, but that’s just one specific metal. Opening up whole new sets of possibilities always creates progress.