Because NASA is conservative as hell . And it has to be . It ' s budget is so small , its mission so bloated and its direction so politicized that any misstep can have catastrophic implications for next decade . That ' s why NASA nixes the most innovative proposals and sticks to same approach since the 70 s . For example , inflatable modules for space stations . Cheaper , lighter , easy to manufacture and replace , oh and also from the 60 s . The BEAM module attached to ISS right now is using materials known since 70 s and module several times larger TransHab was supposed to be part of ISS from the start . Until it got cut by Congress , BTW both parties sponsored the bill and both parties voted for it . Edit Same goes for nonchemical propulsion and nonsolar energy sources . First US nuclear reactor in space was launched in 1965 , and it was also the last . Imagine the missions that would be possible without such stringent energy constraints we have today .