Some of it has to do with the mathematical realities of the solar system. Mars will be closest to Earth in 2018, but if we can't get there that quickly, after that Mars will not come nearly as close to Earth through the 20's until we get to the 30's[0]
An unmanned probe can take an extra month or year to get there with no changes to the actual mission as long as the path is fuel efficient.
For human cargo, the time difference is as important as the fuel expenditure - extra life support supplies can be heavier than extra fuel to make the same flight faster, or the need for much longer life support may make the whole thing impossibly heavy.
[0]http://cseligman.com/text/planets/marsoppositions.htm