The problem is that there seems to be a ceiling to the amount of money the US is willing to spend on space exploration and NASA's current budget is hovering around it. Unless something dramatic happens (e.g. Curiosity finding ancient ruins on Mars, or the Chinese declaring a Race to Mars), this is not going to change in the near and not-so-near future. One can't "budget payloads later" if the budget is not big enough for them.
(English is not my first language, so please excuse any mistakes and unintended ambiguities.)