I found this question, and I was wondering why the ISS is constantly manned (Except for some hours during particular situations). Is that a technical problem? Is it to guarantee safety? Or just because it would be a waste of money to leave it empty?
The ISS is a science laboratory -- a National Lab in fact. Leaving it uncrewed would take away a huge portion of the science productivity without substantially impacting the cost to operate it.
Why is it constantly crewed? Because anything else would be a phenomenal waste of resources.
Because the official United States Government policy reason for funding the International Space Station is to (bold-facing mine):
Develop and operate the International Space Station to support activities requiring the unique attributes of humans in space and establish a permanent human presence in Earth orbit. The International Space Station will support future decisions on the feasibility and desirability of conducting further human exploration activities.
(Presidential Decision Directive I National Security Council 49, "Fact Sheet: National Space Policy," White House National Science and Technology Council, 19 September 1996.) quoted in The Changing Purpose of the Space Station
Note that the entire rationale for US involvement in the project is directly linked to humans in space. If the station were to be operated unmanned, the entire reason for its existence would be null and void.
Because it might be more costly not to have people in it.
The experience from previous space stations was that when a minor issue occur on a space station, it can be easily fixed and detected.
If nobody's here, it can in turn become more expensive to fix.
Salyut had such kind of issues: something broke down that prevented the batteries to be charged. It caused the whole station to freeze over, prompting more difficult and extensive repairs that what could've been done if the station was manned at that time.
I see a few people have answered that it would be wasteful to have it empty, and that is why. However both the Skylab and Mir space stations had vacant periods between expeditions to them. So, that can't be the only reason.
So, I think that the reason it has been manned continuously is that it has been possible to do so. There have been more regular manned flights and supply launches since both the USA and Russia have been launching them. (though manned launches have only been from Russia since the shuttle flights stopped).